Database Users

[Q&A] MultiSystem for Android

MultiSystem is a powerful tool for locked- and unlocked-bootloader Android devices with many features that at least includes the following:
Keeps stock system partition safe/rooted
Permenant root survival with proper use
MultiROM support via virtual ROMs
Unlimited number of virtual ROMs
Booting options to choose stock, primary, or secondary virtual ROM
Any of the virtual ROMs can work as a recovery replacement
Flashing multiple ROMs at the same time without a reboot
Ability to create/install ROMs on Linux to microSD card
Great performance & battery life on virtual ROMs
Recovery solution to install ROMs or Mods
Easy upgrade to newer versions of Android
Ability to safely apply OTA updates to virtual system
Permissive SELinux and other kernel tweaks
Safe flashing that doesn't trip KNOX flag on Samsung devices
Wrapper script runs via ADB or a Terminal Emulator on device
APK to manage all MultiSystem functions with a nice UI and extra options
Management for the best performance & user experience
Support for all Android devices with microSD card
Portability to almost all devices
Compatibility with all Android versions
Click to expand...
Click to collapse
Q&A​
What is the concept behind MultiSystem?
It runs virtual Android ROMs on microSD, like booting multiple systems on a PC from different partitions/disks. So, your stock system partition is kept safe/rooted. It won't affect performance or anything (might even be better on the virtual system if you've high quality microSD & the device supports its speed). Also, you can freely modify any of the virtual systems & in the worst case, reboot the safe stock system or another working virtual system to recover. So, no root loss or potential damage to the original device partitions.
Click to expand...
Click to collapse
Is it a recovery or an APK tool?
It's a shell script that hijacks system at early boot & force Android to boot from the stock system partition or a virtual system IMG & an APK that manages all booting options, virtual ROMs, and works as a recovery replacement + extra features...
Click to expand...
Click to collapse
Does it work as a recovery replacement?
It IS a POWERFUL recovery replacement. You can do whatever you do in recovery with the APK. HOW? recovery does its magic b/c it doesn't depend on the system & has its own kernel/ramdisk. In MultiSystem, you can boot a virtual ROM from extSD that sure doesn't depend on stock system partition or any of the other virtual ROMs (it does depend on the kernel, which you can't flash on locked devcies anyway). Hence, install, backup, restore, ... & all recovery functions are all possible +++ more features since you're running a full ROM not just a recovery ramdisk like Safestrap.
Bottom Line: I think it's the best & most convenient recovery replacement ever for locked devices & it can also attract unlocked devices for the powerful features, MultiROM, and recovery from within ROM.
Click to expand...
Click to collapse
Can I use FlashFire along with MultiSystem?
Yes. MultiSystem is compatible with FlashFire & fully supports it on stock & virtual ROMs. So, you can use both/any of them for flashing to either a stock or virtual ROM. However, it's recommended to use MultiSystem when flashing to the stock system partition (shouldn't be needed anyway since you can always be safe & flash to your old/new virtual ROMs).
Click to expand...
Click to collapse
Does MultiSystem require FlashFire?
No, MultiSystem doesn't require FlashFire. They're fully combatible though.
Click to expand...
Click to collapse
Would the virtual ROM we install be exactly the one in the stock slot?
In MultiSystem APK, you can create a virtual ROM from stock system, a copy from other virtual ROM, a new IMG, a dev-provided ROM, a flashable .ZIP, ... etc. Literally, your virtual ROMs can be any stock or custom ROM that's compatible with your firmware/kernel.
Click to expand...
Click to collapse
How can it run virtual ROMs from external microSD card?
External MicroSD will be formated into 2 partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
It'll hijack the system & boot a virtual system from the 2nd partition. The 1st partition will be automatically detected as your extSD.
Click to expand...
Click to collapse
Can I run unrooted virtual ROM for work apps or any other reason?
Yes. You can add unrooted virtual ROM & reboot to it via MultiSystem APK.
Click to expand...
Click to collapse
How do you boot back into a different ROM?
MultiSystem APK manages all functions including ROM activation & reboot to current system, another stock/virtual system, download mode, recovery, ... etc.
Click to expand...
Click to collapse
Will it be OK to still store media like movies/photos/music to extSD?
100% OK; That's my setup a few months ago. 2 virtual ROMs in the SECOND extSD partition in EXT4 format while all personal data are stored on the FIRST extSD partition in exFAT or FAT32 format... TWO COMPLETELY DIFFERET PARTITIONS.
Click to expand...
Click to collapse
How much space are we going to have for virtual ROMs?
The size of the 2nd partition is optional (> 4GB) for your ROMs, but here is an estimated sizes:
1 Virtual ROM Uncompressed = ~2.7 GB ---> ready for running
1 Virtual ROM Compressed = ~1.5 GB ---> for full ROM backups
I'd say better allocate 4 GB for each ROM you plan to run. If you just need one virtual ROM to keep stock system safe, 4 GB 2nd extSD partition is enough; The remaining space is allocated for the 1st extSD partition as your external storage.
For me, I run Linux too from extSD via MultiSystem. So, I've 64 GB extSD card with two partitions 32 GB each.
Click to expand...
Click to collapse
Can I clear up space on an existing SD card and partition it while full or will the entire card need to be wiped and partitioned from scratch?
You need to backup all your files; it'll be wiped & repartitioned.
Click to expand...
Click to collapse
How can I swap microSD cards & be able to run virtual ROMs?
You can swap microSD cards as you wish provided that the device is powered off; don't remove the microSD card when running a virtual ROM. If the new microSD card doesn't include a 2nd parition of available virtual ROMs, the device will boot directly to the stock system.
Click to expand...
Click to collapse
Is there a specific sd card you recommended for this?
I personally have two microSD cards:
SanDisk Extreme Plus 64GB (Up to 80MB/s read speed)
Samsung 64GB PRO (Up to 90MB/s read speed)
You don't have to change your microSD card for MultiSystem; any card you use on your device should work just fine. The need for more speed is relevant when the device supports that speed & if you're going to buy a new card anyway that you may use with a newer device later.
Click to expand...
Click to collapse
Can I copy virtual ROMs to a new microSD card?
Yes. I'll add a feature for swapping microSD cards so that you can backup/restore virtual ROMs from/to the current extSD to/from internal storage as follows:
power off device
use MultiSystem APK to backup your virtual ROMs
insert the new properly formatted microSD,
power on device (it'll boot to stock system)
use MultiSystem APK to restore your virtual ROMs
use MultiSystem APK to activate one of your virtual ROMs
use MultiSystem APK to reboot to any of your ROMs
Click to expand...
Click to collapse
What about other data/cache partitions and internal storage?
Only system img's are in the extSD. All ROMs share all other partitions. This substantially improves the performance & you won't notice any difference between your stock & virtual ROMs. The reason for performance improvement is that EXT4 loop devices are very fast in reading but not in writing. Your system partition is read-only while data (for example) is read write & cache IMGs cause problems like Safestrap issues on ROM slots. Also, you don't have to worry about switching data/settings between ROMs (they're shared), but you just need to regularly backup your important data (which is healthy anyway).
Click to expand...
Click to collapse
Can your elaborate where data is stored?
The userdata partition is also shared; so, you'll have access to all your FULL storage partitions & all apps/data similarly on either stock or virtual ROMs. This also solves the Safestrap issue of having less storage on ROM slots...
Click to expand...
Click to collapse
Will mSDcard incur a significant performance penalty on some devices?
there's no diffrerence between virtual & stock ROMs in terms of performance & battery life. The reason is simple: loop devices associated with the READ-ONLY system IMG mounted from EXT4 partition using a high-quality microSD card IS very fast more than enough.
The read speed is faster than the device can operate anyway + the exact same device should perform on the lowest speed when reading/writing from/to the FAT/FAT32/ExFAT extSD card (where you store your files or even move apps!!!) anyway, which is much slower than the read speed of a loop device mounted from EXT4 partition.
That's why data partition is shared for many reasons, including the poor READ/WRITE performance.
Click to expand...
Click to collapse
If virtual systems are read only, how do we modify them? Do we have to boot to another multisystem rom to modify a virtual rom?
The stock system partition is mounted by default read only & so are the virtual systems. To modify a stock/virtual system, the MultiSystem APK remounts them read/write. You can modify the currently running virtual system, copy it & modify the copy, modify another stock/virtual system.
Click to expand...
Click to collapse
How is a corrupted virtual rom handled? Does it see it's bad and default to stock system?
At early boot, MultiSystem checks for the microSD & active virtual ROM to boot it. There's a boot menu that gives you options to select a stock/virtual system, but it crashes on LP. I'm debugging it, but all functions won't be affected if I removed it. To fail safe, you can remove the microSD card to boot to stock system & restore/repair your virtual ROMs.
UPDATE1: MultiSystem v1.0.1 now allows you to also switch to stock system on boot to repair corrupted virtual IMGs or any other reasons. More options will be added during boot to ultimately select another virtual system if the active IMG is not booting normally (e.g., bootloop after applying a mod or flashing a bad .ZIP).
UPDATE2: Now, on boot, you can choose from two primary/secondary virtual ROM or stock ROM. Flashing multiple ROMs at the same time without a reboot is now possible.
Click to expand...
Click to collapse
How to check if an IMG is corrupted using MultiSystem status?
Code:
Current System IMG: Test_Rom.img
Current System DEV: [B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
When you see "/dev/block/mmcblk0p23"; it's the original system partition; so MultiSystem failed to boot Test_Rom.img, but it should be your current system.
So, the check is simple based on "Current System Device":
/dev/block/mmcblk0p23 = Stock System Partition
/dev/block/loop0 = Virtual System IMG
Note: The block device number (mmcblk0p23) may vary per device & per variant !
Click to expand...
Click to collapse
Does android do any maintenance whatsoever on stored data within /data or external sd? So if I have an app installed on 1 system and not on another system will android see it and clear the data?
No, all storage partitions are shared between ROMs. If you installed an app, it'll be availabe for all of them. Since on locked devcies we're limited to stock manufacturer-based ROMs, this makes the switch between ROMs very convinient (you don't have to worry about your changes/data/setup & storage space on the another ROM; all ROMs share everything except system). However, you should make regular backups in case a virtual ROM (probably with unsafe mods) results in bootloop due to your user data. In this case, it's safe to wipe data & selectively restore apps/data from backup(s). Another advantage of sharing all storage partitions is that your messages/emails/etc received on a virtual ROM are immediated synced (actually shared) to the other ROMs.
Click to expand...
Click to collapse
Will anything like Xposed modify the virtual ROM system IMG as opposed to the stock system IMG?
When you run a Virtual System, everything incldung kernel & apps are hijacked to speak to it as the original system.
Click to expand...
Click to collapse
Can we install AOSP ROMs on locked devices?
You can only install stock/manufacturer-based ROMs on locked devices while unlocked devices can use kexec or flash the required kernel to boot any AOSP/Stock ROMs. I've got a Note 4 Developer Edition & a lot of development is planned to go there (thanks to the unlocked bootloader!) More devices will get supported including unlocked TMO & international variants after adding more features untilizing the unlocked bootloader with kexec'd kernels.
Click to expand...
Click to collapse
Are there limitations to the combinations of ROMs that can be loaded on the "stock" and "virtual" slots? Can you mix KK and LP?
Yes, if they can run on the same kernel. LP won't run on KK kernels & so, you'd have to upgrade the firmware anyway. As for running mixed compatible Android versions, this is possible but your'd have to backup your data before switching ROMs; if it cause no issues, enjoy smooth switch & if it doesn't, do factory reset in recovery & restore your data backup. Backups via MultiSystem are painless.
Click to expand...
Click to collapse
Are applications installed once for each ROM slot that has that applicaiton installed, or can I share a game across ROMs (for instance?)
Everything is shared between ROMs, which is very good for storage & for easy switching. Just make regular backups of your sensitive data.
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Click to collapse
How there are no performance hits while internal storage memory was much faster than any microSD technology?
Read speeds from microSD is very fast compared to write speeds & since virtual ROMs are actually a virtual read-only systems (hence, MultiSystem), they provide a high performance. Moreover, again, read speeds from EXT4 loop devices are higher compared to physical partitions. They're very bad in writing, which we don't need for the read-only "system".
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Click to collapse
Is there a preferred "daily driver" ROM that should be installed in the stock slot?
Uses a stock ODEXED ROM on stock slot for better stability!
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Click to collapse
Is it based off of Safestrap?
Short answer NO. I've been working on MultiSystem & Safestrap for ~7 months. Earlier versions of MultiSystem (called, JasmineREC) was based on Safestrap, but it failed to support newer versions of Android mainly due to TWRP changes in the graphics/UI libraries that cause segmentation fault & the stock kernel framebuffer issues. Then, I decided to find another solution. However, the basic idea of system hijack is powered by Safestrap (or 2nd-init recoveries in general) & all the work done by @Hashcode is GREATLY appreciated.
Click to expand...
Click to collapse
How can it overwrite system files while running?
MultiSystem allows you to install safe mod's or a ROM in full or OTA-like update. It's strongly recommended to install .ZIP files NOT to the current system, b/c some files can not be overwritten while running. So, you can use backup function to copy the current system & install to the new img or any of your other virtual systems. You'll have several options to activate a virtual img & reboot directly to stock system, any virtual img you've activated, quick reboot, Download/bootloader, recovery,... etc.
Click to expand...
Click to collapse
How would I benefit from it if I'm only running Stock ROM or would there be no point for me to install it?
If you run a ROM on stock system, you're vulnerable to root loss unless/untill a new rooting method for LP comes out. MultiSystem gives you the option to run safe-to-mod virtual ROMs + recovery replacement + extra features.
Click to expand...
Click to collapse
Is there a way to convert a normal ROM .ZIP into MultiSystem .IMG?
Create or copy any of your IMGs, activate it & reboot to the active IMG! Then, use FlashFire to flash the ZIP file. However, the updater-script should be safe/compatible. Some devs mount the phyical partition, which will redirect everything to it!!
For example:
Code:
mount(“ext4″, “EMMC”, “/dev/block/mmcblk0p23″, “/system”);
will mount the original system partition; while
Code:
run_program("/sbin/mount", "-t", "auto", "/system");
will mount the current system (stock or virtual). This is recommended/safe.
Click to expand...
Click to collapse
Would a KitKat ROM work with multisystem even though my stock is Lollipop?
Any ROM requires a compatible kernel & modem. So, running KK ROMs requires flashing KK firmware (namely, kernel & modem). This may work with MultiSystem on other devices, especially if the bootlpoader is unlocked. For example, I plan to add features for Note 4 DevED to allow different Android versions (including AOSP, manufacturer-based, & probably Linux systems) by utilizing kernel swapping or execution.
Click to expand...
Click to collapse
When MultiSystem comes out will it be open sourced?
Most probably, haven't decided yet!
Anyway, here's the repository on GitHub: https://github.com/hsbadr/MultiSystem
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Click to collapse
Click to expand...
Click to collapse

Video Tutorials
A quick preview of MultiSystem v1.0 tested on Lollipop for VZW Note 3. The video has been captured on a stable virtual ROM of JasmineROM v5.0.1. It's FULLY compatible with FlashFire on virtual/stock systems. More devices will get supported as well, after required testing.
Facebook: https://www.facebook.com/hsbadr/videos/vb.331488823689599/428178174020663

How to check if you are running a Stock/Virtual System?
There're many ways to check whether you're running a Stock or Virtual system. MultiSystem app should include this simple check at some point. That's important to avoint ruining the Stock system & keep it safe. To make it clear to NOOBZ & anyone who's requesting "another" proof even though I owe hime nothing. Very weird!
Anyway, BusyBox mountpoint applet can print the current block/device mounted to /system mountpoint by running the following command:
Code:
busybox mountpoint -n /system
The stock system is mounts the original system partition:
Code:
[B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
while the virtual system mounts a loop device associated with a system IMG:
Code:
[B][COLOR="Blue"]/dev/block/loop0[/COLOR][/B]
Here're two videos for both stock & virtual systems...
UPDATE:
Now, you could run the following command to print the current system (stock or virtual) and the system device (physical partition or loop device):
Code:
MultiSystem status
Note: The block device number (mmcblk0p23) may vary per device & per variant !

How to repartition microSD card for MultiSystem?
You can use any tool/program for partitioning on Android, Linux, Mac, or Windows. For example, MiniTool Partition Wizard is a good partitioning tool for Windows. So, let's use it for this task. Simply, you need to follow this PDF tutorial (thanks to @carl1961). In sum:
Step 1: delete old partitions on SD card
Step 2: create FAT32 PRIMARY partition
Step 3: create EXT4 PRIMARY partition
Then, apply changes (note that the program UI may get changed in newer versions).
Notes:
This partitioning tutorial doesn't create PRIMARY partitions (it creates logical partitions). So, you need to change "Create As" from "Logical" to "Primary" when creatig a partition.
The sizes of the two partitions are arbitrary depending on number of ROMs you plan to install on the 2nd EXT4 partition.
The 1st partition (check size) is automatically detected as your external storage
In Terminal Emulator or ADB shell, check the existence of the two partitions by running the following command (in red):
Code:
[email protected]:/ # [COLOR="Red"]ls -l /dev/block/platform/msm_sdcc.3/[/COLOR]
drwxr-xr-x root root 2015-05-02 21:08 by-num
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1 -> /dev/block/mmcblk1
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p1 -> [COLOR="Blue"]/dev/block/mmcblk1p1[/COLOR]
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p2 -> [COLOR="Blue"]/dev/block/mmcblk1p2[/COLOR]
/dev/block/mmcblk1p1 is mounted by Android as your external storage.
/dev/block/mmcblk1p2 is NOT mounted & will be your MultiSystem partition.
Click to expand...
Click to collapse

How to check microSD card partitions for MultiSystem?
You need to correctly repartition microSD card into two partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
Use the directions in this post!
You should check your 2nd SD partition in EXT4 format mounted to /MultiSystem:
check that the /MultiSystem directory exists after a reboot
check that the 2nd SD partition (/dev/block/mmcblk1p2) is mounted to /MultiSystem by running the following command in Terminal Emulator or ADB shell:
Code:
mount | grep /MultiSystem
The output should be:
Code:
/dev/block/mmcblk1p2 /MultiSystem ext4 rw,seclabel,relatime,data=ordered 0 0

How to check MultiSystem Installation?
The 1st thing to do after installing MultiSystem is to check the /MultiSystem directory & its contents (it shouldn't be empty!). Then, check usage by running the following commands in Terminal Emulator or ADB shell:
Code:
su
bash
MultiSystem
If it retuns "MultiSystem not found" or permission denied, try to use open MultiSystem app to Update Configurations & try again. If this does't fix it, try the following command:
Code:
/MultiSystem/bin/MultiSystem
This should work if you've MultiSystem binaries installed in (extracted to) /MultiSystem directory. If so, you can create a symlink in /system/xbin as follows:
Code:
mount -o remount,rw /system
ln -sv /MultiSystem/bin/MultiSystem /system/xbin/MultiSystem
Then, test it by running:
Code:
MultiSystem
The last thing before using it is to check the boot options: reboot & monitor the GREEN LED indicator for 3 seconds (change in the app) , which give you the following options:
Volume UP = Primary virtual ROM
Volume DOWN = Secondary virtual ROM
HOME KEY = Stock System
Sure, you should have installed one or more virtual ROMs.
Backup & restore or creating/installing a virtual ROM are easy as copy & paste: all img's will be at
Code:
/MultiSystem/img/system
To backup a virtual/stock system, you have many options:
Use create function to create from stock system
Use copy function to copy the IMG
Copy & paste with a new name
Use FlashFire (fully supported on virtual/stock ROMs)
...
If you've IMG mounting issues, run the following commands:
Code:
mount -o remount,rw /system
busybox ln -sv /proc/self/mounts /system/etc/mtab
If this doesn't help, try mounting from Terminal Emulator or ADB shell after selecting the IMG in MultiSystem app, by running the following command:
Code:
MultiSystem mount virtual

MultiSystem Video Tutorial
Thanks To: @Tomsgt , aka RootJunky
Don't forget to subscribe & like the video to show appreciation of his great effort & time spent in making the video :highfive::good:

hi, thanks for the work! i have a problem, i have MultiSystem installed, when i put MultiSystem status in the terminal it appears same as the tutorial video, now when i use the "Create Virtual IMG" button, the led goes green, but after 5 minutes turns off. then i cannot select the system img in the "Primary System IMG" section, and when i go to the MultiSystem directory and search for the img on "img/system/" there is a img file but without size, the size says something like "-1400897536".
thanks for your help!
Im using a Samsung Galaxy Sgh-i337 with locked bootloader, runing multisystem on stock Lollipop 5.0.1 rooted. i have busybox.

emptyPD said:
hi, thanks for the work! i have a problem, i have MultiSystem installed, when i put MultiSystem status in the terminal it appears same as the tutorial video, now when i use the "Create Virtual IMG" button, the led goes green, but after 5 minutes turns off. then i cannot select the system img in the "Primary System IMG" section, and when i go to the MultiSystem directory and search for the img on "img/system/" there is a img file but without size, the size says something like "-1400897536".
thanks for your help!
Im using a Samsung Galaxy Sgh-i337 with locked bootloader, runing multisystem on stock Lollipop 5.0.1 rooted. i have busybox.
Click to expand...
Click to collapse
i solved the problem using "MultiSystem update" in the terminal emulator. after that i was able to view the IMGs

[Q&A] MultiSystem for Android

MultiSystem is a powerful tool for locked- and unlocked-bootloader Android devices with many features that at least includes the following:
Keeps stock system partition safe/rooted
Permenant root survival with proper use
MultiROM support via virtual ROMs
Unlimited number of virtual ROMs
Booting options to choose stock, primary, or secondary virtual ROM
Any of the virtual ROMs can work as a recovery replacement
Flashing multiple ROMs at the same time without a reboot
Ability to create/install ROMs on Linux to microSD card
Great performance & battery life on virtual ROMs
Recovery solution to install ROMs or Mods
Easy upgrade to newer versions of Android
Ability to safely apply OTA updates to virtual system
Permissive SELinux and other kernel tweaks
Safe flashing that doesn't trip KNOX flag on Samsung devices
Wrapper script runs via ADB or a Terminal Emulator on device
APK to manage all MultiSystem functions with a nice UI and extra options
Management for the best performance & user experience
Support for all Android devices with microSD card
Portability to almost all devices
Compatibility with all Android versions
Click to expand...
Click to collapse
Q&A​
What is the concept behind MultiSystem?
It runs virtual Android ROMs on microSD, like booting multiple systems on a PC from different partitions/disks. So, your stock system partition is kept safe/rooted. It won't affect performance or anything (might even be better on the virtual system if you've high quality microSD & the device supports its speed). Also, you can freely modify any of the virtual systems & in the worst case, reboot the safe stock system or another working virtual system to recover. So, no root loss or potential damage to the original device partitions.
Click to expand...
Click to collapse
Is it a recovery or an APK tool?
It's a shell script that hijacks system at early boot & force Android to boot from the stock system partition or a virtual system IMG & an APK that manages all booting options, virtual ROMs, and works as a recovery replacement + extra features...
Click to expand...
Click to collapse
Does it work as a recovery replacement?
It IS a POWERFUL recovery replacement. You can do whatever you do in recovery with the APK. HOW? recovery does its magic b/c it doesn't depend on the system & has its own kernel/ramdisk. In MultiSystem, you can boot a virtual ROM from extSD that sure doesn't depend on stock system partition or any of the other virtual ROMs (it does depend on the kernel, which you can't flash on locked devcies anyway). Hence, install, backup, restore, ... & all recovery functions are all possible +++ more features since you're running a full ROM not just a recovery ramdisk like Safestrap.
Bottom Line: I think it's the best & most convenient recovery replacement ever for locked devices & it can also attract unlocked devices for the powerful features, MultiROM, and recovery from within ROM.
Click to expand...
Click to collapse
Can I use FlashFire along with MultiSystem?
Yes. MultiSystem is compatible with FlashFire & fully supports it on stock & virtual ROMs. So, you can use both/any of them for flashing to either a stock or virtual ROM. However, it's recommended to use MultiSystem when flashing to the stock system partition (shouldn't be needed anyway since you can always be safe & flash to your old/new virtual ROMs).
Click to expand...
Click to collapse
Does MultiSystem require FlashFire?
No, MultiSystem doesn't require FlashFire. They're fully combatible though.
Click to expand...
Click to collapse
Would the virtual ROM we install be exactly the one in the stock slot?
In MultiSystem APK, you can create a virtual ROM from stock system, a copy from other virtual ROM, a new IMG, a dev-provided ROM, a flashable .ZIP, ... etc. Literally, your virtual ROMs can be any stock or custom ROM that's compatible with your firmware/kernel.
Click to expand...
Click to collapse
How can it run virtual ROMs from external microSD card?
External MicroSD will be formated into 2 partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
It'll hijack the system & boot a virtual system from the 2nd partition. The 1st partition will be automatically detected as your extSD.
Click to expand...
Click to collapse
Can I run unrooted virtual ROM for work apps or any other reason?
Yes. You can add unrooted virtual ROM & reboot to it via MultiSystem APK.
Click to expand...
Click to collapse
How do you boot back into a different ROM?
MultiSystem APK manages all functions including ROM activation & reboot to current system, another stock/virtual system, download mode, recovery, ... etc.
Click to expand...
Click to collapse
Will it be OK to still store media like movies/photos/music to extSD?
100% OK; That's my setup a few months ago. 2 virtual ROMs in the SECOND extSD partition in EXT4 format while all personal data are stored on the FIRST extSD partition in exFAT or FAT32 format... TWO COMPLETELY DIFFERET PARTITIONS.
Click to expand...
Click to collapse
How much space are we going to have for virtual ROMs?
The size of the 2nd partition is optional (> 4GB) for your ROMs, but here is an estimated sizes:
1 Virtual ROM Uncompressed = ~2.7 GB ---> ready for running
1 Virtual ROM Compressed = ~1.5 GB ---> for full ROM backups
I'd say better allocate 4 GB for each ROM you plan to run. If you just need one virtual ROM to keep stock system safe, 4 GB 2nd extSD partition is enough; The remaining space is allocated for the 1st extSD partition as your external storage.
For me, I run Linux too from extSD via MultiSystem. So, I've 64 GB extSD card with two partitions 32 GB each.
Click to expand...
Click to collapse
Can I clear up space on an existing SD card and partition it while full or will the entire card need to be wiped and partitioned from scratch?
You need to backup all your files; it'll be wiped & repartitioned.
Click to expand...
Click to collapse
How can I swap microSD cards & be able to run virtual ROMs?
You can swap microSD cards as you wish provided that the device is powered off; don't remove the microSD card when running a virtual ROM. If the new microSD card doesn't include a 2nd parition of available virtual ROMs, the device will boot directly to the stock system.
Click to expand...
Click to collapse
Is there a specific sd card you recommended for this?
I personally have two microSD cards:
SanDisk Extreme Plus 64GB (Up to 80MB/s read speed)
Samsung 64GB PRO (Up to 90MB/s read speed)
You don't have to change your microSD card for MultiSystem; any card you use on your device should work just fine. The need for more speed is relevant when the device supports that speed & if you're going to buy a new card anyway that you may use with a newer device later.
Click to expand...
Click to collapse
Can I copy virtual ROMs to a new microSD card?
Yes. I'll add a feature for swapping microSD cards so that you can backup/restore virtual ROMs from/to the current extSD to/from internal storage as follows:
power off device
use MultiSystem APK to backup your virtual ROMs
insert the new properly formatted microSD,
power on device (it'll boot to stock system)
use MultiSystem APK to restore your virtual ROMs
use MultiSystem APK to activate one of your virtual ROMs
use MultiSystem APK to reboot to any of your ROMs
Click to expand...
Click to collapse
What about other data/cache partitions and internal storage?
Only system img's are in the extSD. All ROMs share all other partitions. This substantially improves the performance & you won't notice any difference between your stock & virtual ROMs. The reason for performance improvement is that EXT4 loop devices are very fast in reading but not in writing. Your system partition is read-only while data (for example) is read write & cache IMGs cause problems like Safestrap issues on ROM slots. Also, you don't have to worry about switching data/settings between ROMs (they're shared), but you just need to regularly backup your important data (which is healthy anyway).
Click to expand...
Click to collapse
Can your elaborate where data is stored?
The userdata partition is also shared; so, you'll have access to all your FULL storage partitions & all apps/data similarly on either stock or virtual ROMs. This also solves the Safestrap issue of having less storage on ROM slots...
Click to expand...
Click to collapse
Will mSDcard incur a significant performance penalty on some devices?
there's no diffrerence between virtual & stock ROMs in terms of performance & battery life. The reason is simple: loop devices associated with the READ-ONLY system IMG mounted from EXT4 partition using a high-quality microSD card IS very fast more than enough.
The read speed is faster than the device can operate anyway + the exact same device should perform on the lowest speed when reading/writing from/to the FAT/FAT32/ExFAT extSD card (where you store your files or even move apps!!!) anyway, which is much slower than the read speed of a loop device mounted from EXT4 partition.
That's why data partition is shared for many reasons, including the poor READ/WRITE performance.
Click to expand...
Click to collapse
If virtual systems are read only, how do we modify them? Do we have to boot to another multisystem rom to modify a virtual rom?
The stock system partition is mounted by default read only & so are the virtual systems. To modify a stock/virtual system, the MultiSystem APK remounts them read/write. You can modify the currently running virtual system, copy it & modify the copy, modify another stock/virtual system.
Click to expand...
Click to collapse
How is a corrupted virtual rom handled? Does it see it's bad and default to stock system?
At early boot, MultiSystem checks for the microSD & active virtual ROM to boot it. There's a boot menu that gives you options to select a stock/virtual system, but it crashes on LP. I'm debugging it, but all functions won't be affected if I removed it. To fail safe, you can remove the microSD card to boot to stock system & restore/repair your virtual ROMs.
UPDATE1: MultiSystem v1.0.1 now allows you to also switch to stock system on boot to repair corrupted virtual IMGs or any other reasons. More options will be added during boot to ultimately select another virtual system if the active IMG is not booting normally (e.g., bootloop after applying a mod or flashing a bad .ZIP).
UPDATE2: Now, on boot, you can choose from two primary/secondary virtual ROM or stock ROM. Flashing multiple ROMs at the same time without a reboot is now possible.
Click to expand...
Click to collapse
How to check if an IMG is corrupted using MultiSystem status?
Code:
Current System IMG: Test_Rom.img
Current System DEV: [B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
When you see "/dev/block/mmcblk0p23"; it's the original system partition; so MultiSystem failed to boot Test_Rom.img, but it should be your current system.
So, the check is simple based on "Current System Device":
/dev/block/mmcblk0p23 = Stock System Partition
/dev/block/loop0 = Virtual System IMG
Note: The block device number (mmcblk0p23) may vary per device & per variant !
Click to expand...
Click to collapse
Does android do any maintenance whatsoever on stored data within /data or external sd? So if I have an app installed on 1 system and not on another system will android see it and clear the data?
No, all storage partitions are shared between ROMs. If you installed an app, it'll be availabe for all of them. Since on locked devcies we're limited to stock manufacturer-based ROMs, this makes the switch between ROMs very convinient (you don't have to worry about your changes/data/setup & storage space on the another ROM; all ROMs share everything except system). However, you should make regular backups in case a virtual ROM (probably with unsafe mods) results in bootloop due to your user data. In this case, it's safe to wipe data & selectively restore apps/data from backup(s). Another advantage of sharing all storage partitions is that your messages/emails/etc received on a virtual ROM are immediated synced (actually shared) to the other ROMs.
Click to expand...
Click to collapse
Will anything like Xposed modify the virtual ROM system IMG as opposed to the stock system IMG?
When you run a Virtual System, everything incldung kernel & apps are hijacked to speak to it as the original system.
Click to expand...
Click to collapse
Can we install AOSP ROMs on locked devices?
You can only install stock/manufacturer-based ROMs on locked devices while unlocked devices can use kexec or flash the required kernel to boot any AOSP/Stock ROMs. I've got a Note 4 Developer Edition & a lot of development is planned to go there (thanks to the unlocked bootloader!) More devices will get supported including unlocked TMO & international variants after adding more features untilizing the unlocked bootloader with kexec'd kernels.
Click to expand...
Click to collapse
Are there limitations to the combinations of ROMs that can be loaded on the "stock" and "virtual" slots? Can you mix KK and LP?
Yes, if they can run on the same kernel. LP won't run on KK kernels & so, you'd have to upgrade the firmware anyway. As for running mixed compatible Android versions, this is possible but your'd have to backup your data before switching ROMs; if it cause no issues, enjoy smooth switch & if it doesn't, do factory reset in recovery & restore your data backup. Backups via MultiSystem are painless.
Click to expand...
Click to collapse
Are applications installed once for each ROM slot that has that applicaiton installed, or can I share a game across ROMs (for instance?)
Everything is shared between ROMs, which is very good for storage & for easy switching. Just make regular backups of your sensitive data.
Click to expand...
Click to collapse
How there are no performance hits while internal storage memory was much faster than any microSD technology?
Read speeds from microSD is very fast compared to write speeds & since virtual ROMs are actually a virtual read-only systems (hence, MultiSystem), they provide a high performance. Moreover, again, read speeds from EXT4 loop devices are higher compared to physical partitions. They're very bad in writing, which we don't need for the read-only "system".
Click to expand...
Click to collapse
Is there a preferred "daily driver" ROM that should be installed in the stock slot?
Uses a stock ODEXED ROM on stock slot for better stability!
Click to expand...
Click to collapse
Is it based off of Safestrap?
Short answer NO. I've been working on MultiSystem & Safestrap for ~7 months. Earlier versions of MultiSystem (called, JasmineREC) was based on Safestrap, but it failed to support newer versions of Android mainly due to TWRP changes in the graphics/UI libraries that cause segmentation fault & the stock kernel framebuffer issues. Then, I decided to find another solution. However, the basic idea of system hijack is powered by Safestrap (or 2nd-init recoveries in general) & all the work done by @Hashcode is GREATLY appreciated.
Click to expand...
Click to collapse
How can it overwrite system files while running?
MultiSystem allows you to install safe mod's or a ROM in full or OTA-like update. It's strongly recommended to install .ZIP files NOT to the current system, b/c some files can not be overwritten while running. So, you can use backup function to copy the current system & install to the new img or any of your other virtual systems. You'll have several options to activate a virtual img & reboot directly to stock system, any virtual img you've activated, quick reboot, Download/bootloader, recovery,... etc.
Click to expand...
Click to collapse
How would I benefit from it if I'm only running Stock ROM or would there be no point for me to install it?
If you run a ROM on stock system, you're vulnerable to root loss unless/untill a new rooting method for LP comes out. MultiSystem gives you the option to run safe-to-mod virtual ROMs + recovery replacement + extra features.
Click to expand...
Click to collapse
Is there a way to convert a normal ROM .ZIP into MultiSystem .IMG?
Create or copy any of your IMGs, activate it & reboot to the active IMG! Then, use FlashFire to flash the ZIP file. However, the updater-script should be safe/compatible. Some devs mount the phyical partition, which will redirect everything to it!!
For example:
Code:
mount(“ext4″, “EMMC”, “/dev/block/mmcblk0p23″, “/system”);
will mount the original system partition; while
Code:
run_program("/sbin/mount", "-t", "auto", "/system");
will mount the current system (stock or virtual). This is recommended/safe.
Click to expand...
Click to collapse
Would a KitKat ROM work with multisystem even though my stock is Lollipop?
Any ROM requires a compatible kernel & modem. So, running KK ROMs requires flashing KK firmware (namely, kernel & modem). This may work with MultiSystem on other devices, especially if the bootlpoader is unlocked. For example, I plan to add features for Note 4 DevED to allow different Android versions (including AOSP, manufacturer-based, & probably Linux systems) by utilizing kernel swapping or execution.
Click to expand...
Click to collapse
When MultiSystem comes out will it be open sourced?
Most probably, haven't decided yet!
Anyway, here's the repository on GitHub: https://github.com/hsbadr/MultiSystem
Click to expand...
Click to collapse
Click to expand...
Click to collapse

Video Tutorials
A quick preview of MultiSystem v1.0 tested on Lollipop for VZW Note 3. The video has been captured on a stable virtual ROM of JasmineROM v5.0.1. It's FULLY compatible with FlashFire on virtual/stock systems. More devices will get supported as well, after required testing.
Facebook: https://www.facebook.com/hsbadr/videos/vb.331488823689599/428178174020663

How to check if you are running a Stock/Virtual System?
There're many ways to check whether you're running a Stock or Virtual system. MultiSystem app should include this simple check at some point. That's important to avoint ruining the Stock system & keep it safe. To make it clear to NOOBZ & anyone who's requesting "another" proof even though I owe hime nothing. Very weird!
Anyway, BusyBox mountpoint applet can print the current block/device mounted to /system mountpoint by running the following command:
Code:
busybox mountpoint -n /system
The stock system is mounts the original system partition:
Code:
[B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
while the virtual system mounts a loop device associated with a system IMG:
Code:
[B][COLOR="Blue"]/dev/block/loop0[/COLOR][/B]
Here're two videos for both stock & virtual systems...
UPDATE:
Now, you could run the following command to print the current system (stock or virtual) and the system device (physical partition or loop device):
Code:
MultiSystem status
Note: The block device number (mmcblk0p23) may vary per device & per variant !

How to repartition microSD card for MultiSystem?
You can use any tool/program for partitioning on Android, Linux, Mac, or Windows. For example, MiniTool Partition Wizard is a good partitioning tool for Windows. So, let's use it for this task. Simply, you need to follow this PDF tutorial (thanks to @carl1961). In sum:
Step 1: delete old partitions on SD card
Step 2: create FAT32 PRIMARY partition
Step 3: create EXT4 PRIMARY partition
Then, apply changes (note that the program UI may get changed in newer versions).
Notes:
This partitioning tutorial doesn't create PRIMARY partitions (it creates logical partitions). So, you need to change "Create As" from "Logical" to "Primary" when creatig a partition.
The sizes of the two partitions are arbitrary depending on number of ROMs you plan to install on the 2nd EXT4 partition.
The 1st partition (check size) is automatically detected as your external storage
In Terminal Emulator or ADB shell, check the existence of the two partitions by running the following command (in red):
Code:
[email protected]:/ # [COLOR="Red"]ls -l /dev/block/platform/msm_sdcc.3/[/COLOR]
drwxr-xr-x root root 2015-05-02 21:08 by-num
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1 -> /dev/block/mmcblk1
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p1 -> [COLOR="Blue"]/dev/block/mmcblk1p1[/COLOR]
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p2 -> [COLOR="Blue"]/dev/block/mmcblk1p2[/COLOR]
/dev/block/mmcblk1p1 is mounted by Android as your external storage.
/dev/block/mmcblk1p2 is NOT mounted & will be your MultiSystem partition.
Click to expand...
Click to collapse

How to check microSD card partitions for MultiSystem?
You need to correctly repartition microSD card into two partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
Use the directions in this post!
You should check your 2nd SD partition in EXT4 format mounted to /MultiSystem:
check that the /MultiSystem directory exists after a reboot
check that the 2nd SD partition (/dev/block/mmcblk1p2) is mounted to /MultiSystem by running the following command in Terminal Emulator or ADB shell:
Code:
mount | grep /MultiSystem
The output should be:
Code:
/dev/block/mmcblk1p2 /MultiSystem ext4 rw,seclabel,relatime,data=ordered 0 0

How to check MultiSystem Installation?
The 1st thing to do after installing MultiSystem is to check the /MultiSystem directory & its contents (it shouldn't be empty!). Then, check usage by running the following commands in Terminal Emulator or ADB shell:
Code:
su
bash
MultiSystem
If it retuns "MultiSystem not found" or permission denied, try to use open MultiSystem app to Update Configurations & try again. If this does't fix it, try the following command:
Code:
/MultiSystem/bin/MultiSystem
This should work if you've MultiSystem binaries installed in (extracted to) /MultiSystem directory. If so, you can create a symlink in /system/xbin as follows:
Code:
mount -o remount,rw /system
ln -sv /MultiSystem/bin/MultiSystem /system/xbin/MultiSystem
Then, test it by running:
Code:
MultiSystem
The last thing before using it is to check the boot options: reboot & monitor the GREEN LED indicator for 3 seconds (change in the app) , which give you the following options:
Volume UP = Primary virtual ROM
Volume DOWN = Secondary virtual ROM
HOME KEY = Stock System
Sure, you should have installed one or more virtual ROMs.
Backup & restore or creating/installing a virtual ROM are easy as copy & paste: all img's will be at
Code:
/MultiSystem/img/system
To backup a virtual/stock system, you have many options:
Use create function to create from stock system
Use copy function to copy the IMG
Copy & paste with a new name
Use FlashFire (fully supported on virtual/stock ROMs)
...
If you've IMG mounting issues, run the following commands:
Code:
mount -o remount,rw /system
busybox ln -sv /proc/self/mounts /system/etc/mtab
If this doesn't help, try mounting from Terminal Emulator or ADB shell after selecting the IMG in MultiSystem app, by running the following command:
Code:
MultiSystem mount virtual

MultiSystem Video Tutorial
Thanks To: @Tomsgt , aka RootJunky
Don't forget to subscribe & like the video to show appreciation of his great effort & time spent in making the video :highfive::good:

Maybe not ready for the s5
I have installed this on my s5. It's very buggy, at least for me it has just been a world of trouble. I had to restore back to stock eventually. I did not have full permissive Linux either. It was weird multi system was completely installed on my device and I could switch between two roms. But I couldn't find multisystem anywhere in terminal.

Thanks in advance

[Q&A] MultiSystem for Android

MultiSystem is a powerful tool for locked- and unlocked-bootloader Android devices with many features that at least includes the following:
Keeps stock system partition safe/rooted
Permenant root survival with proper use
MultiROM support via virtual ROMs
Unlimited number of virtual ROMs
Booting options to choose stock, primary, or secondary virtual ROM
Any of the virtual ROMs can work as a recovery replacement
Flashing multiple ROMs at the same time without a reboot
Ability to create/install ROMs on Linux to microSD card
Great performance & battery life on virtual ROMs
Recovery solution to install ROMs or Mods
Easy upgrade to newer versions of Android
Ability to safely apply OTA updates to virtual system
Permissive SELinux and other kernel tweaks
Safe flashing that doesn't trip KNOX flag on Samsung devices
Wrapper script runs via ADB or a Terminal Emulator on device
APK to manage all MultiSystem functions with a nice UI and extra options
Management for the best performance & user experience
Support for all Android devices with microSD card
Portability to almost all devices
Compatibility with all Android versions
Click to expand...
Click to collapse
Q&A​
What is the concept behind MultiSystem?
It runs virtual Android ROMs on microSD, like booting multiple systems on a PC from different partitions/disks. So, your stock system partition is kept safe/rooted. It won't affect performance or anything (might even be better on the virtual system if you've high quality microSD & the device supports its speed). Also, you can freely modify any of the virtual systems & in the worst case, reboot the safe stock system or another working virtual system to recover. So, no root loss or potential damage to the original device partitions.
Click to expand...
Click to collapse
Is it a recovery or an APK tool?
It's a shell script that hijacks system at early boot & force Android to boot from the stock system partition or a virtual system IMG & an APK that manages all booting options, virtual ROMs, and works as a recovery replacement + extra features...
Click to expand...
Click to collapse
Does it work as a recovery replacement?
It IS a POWERFUL recovery replacement. You can do whatever you do in recovery with the APK. HOW? recovery does its magic b/c it doesn't depend on the system & has its own kernel/ramdisk. In MultiSystem, you can boot a virtual ROM from extSD that sure doesn't depend on stock system partition or any of the other virtual ROMs (it does depend on the kernel, which you can't flash on locked devcies anyway). Hence, install, backup, restore, ... & all recovery functions are all possible +++ more features since you're running a full ROM not just a recovery ramdisk like Safestrap.
Bottom Line: I think it's the best & most convenient recovery replacement ever for locked devices & it can also attract unlocked devices for the powerful features, MultiROM, and recovery from within ROM.
Click to expand...
Click to collapse
Can I use FlashFire along with MultiSystem?
Yes. MultiSystem is compatible with FlashFire & fully supports it on stock & virtual ROMs. So, you can use both/any of them for flashing to either a stock or virtual ROM. However, it's recommended to use MultiSystem when flashing to the stock system partition (shouldn't be needed anyway since you can always be safe & flash to your old/new virtual ROMs).
Click to expand...
Click to collapse
Does MultiSystem require FlashFire?
No, MultiSystem doesn't require FlashFire. They're fully combatible though.
Click to expand...
Click to collapse
Would the virtual ROM we install be exactly the one in the stock slot?
In MultiSystem APK, you can create a virtual ROM from stock system, a copy from other virtual ROM, a new IMG, a dev-provided ROM, a flashable .ZIP, ... etc. Literally, your virtual ROMs can be any stock or custom ROM that's compatible with your firmware/kernel.
Click to expand...
Click to collapse
How can it run virtual ROMs from external microSD card?
External MicroSD will be formated into 2 partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
It'll hijack the system & boot a virtual system from the 2nd partition. The 1st partition will be automatically detected as your extSD.
Click to expand...
Click to collapse
Can I run unrooted virtual ROM for work apps or any other reason?
Yes. You can add unrooted virtual ROM & reboot to it via MultiSystem APK.
Click to expand...
Click to collapse
How do you boot back into a different ROM?
MultiSystem APK manages all functions including ROM activation & reboot to current system, another stock/virtual system, download mode, recovery, ... etc.
Click to expand...
Click to collapse
Will it be OK to still store media like movies/photos/music to extSD?
100% OK; That's my setup a few months ago. 2 virtual ROMs in the SECOND extSD partition in EXT4 format while all personal data are stored on the FIRST extSD partition in exFAT or FAT32 format... TWO COMPLETELY DIFFERET PARTITIONS.
Click to expand...
Click to collapse
How much space are we going to have for virtual ROMs?
The size of the 2nd partition is optional (> 4GB) for your ROMs, but here is an estimated sizes:
1 Virtual ROM Uncompressed = ~2.7 GB ---> ready for running
1 Virtual ROM Compressed = ~1.5 GB ---> for full ROM backups
I'd say better allocate 4 GB for each ROM you plan to run. If you just need one virtual ROM to keep stock system safe, 4 GB 2nd extSD partition is enough; The remaining space is allocated for the 1st extSD partition as your external storage.
For me, I run Linux too from extSD via MultiSystem. So, I've 64 GB extSD card with two partitions 32 GB each.
Click to expand...
Click to collapse
Can I clear up space on an existing SD card and partition it while full or will the entire card need to be wiped and partitioned from scratch?
You need to backup all your files; it'll be wiped & repartitioned.
Click to expand...
Click to collapse
How can I swap microSD cards & be able to run virtual ROMs?
You can swap microSD cards as you wish provided that the device is powered off; don't remove the microSD card when running a virtual ROM. If the new microSD card doesn't include a 2nd parition of available virtual ROMs, the device will boot directly to the stock system.
Click to expand...
Click to collapse
Is there a specific sd card you recommended for this?
I personally have two microSD cards:
SanDisk Extreme Plus 64GB (Up to 80MB/s read speed)
Samsung 64GB PRO (Up to 90MB/s read speed)
You don't have to change your microSD card for MultiSystem; any card you use on your device should work just fine. The need for more speed is relevant when the device supports that speed & if you're going to buy a new card anyway that you may use with a newer device later.
Click to expand...
Click to collapse
Can I copy virtual ROMs to a new microSD card?
Yes. I'll add a feature for swapping microSD cards so that you can backup/restore virtual ROMs from/to the current extSD to/from internal storage as follows:
power off device
use MultiSystem APK to backup your virtual ROMs
insert the new properly formatted microSD,
power on device (it'll boot to stock system)
use MultiSystem APK to restore your virtual ROMs
use MultiSystem APK to activate one of your virtual ROMs
use MultiSystem APK to reboot to any of your ROMs
Click to expand...
Click to collapse
What about other data/cache partitions and internal storage?
Only system img's are in the extSD. All ROMs share all other partitions. This substantially improves the performance & you won't notice any difference between your stock & virtual ROMs. The reason for performance improvement is that EXT4 loop devices are very fast in reading but not in writing. Your system partition is read-only while data (for example) is read write & cache IMGs cause problems like Safestrap issues on ROM slots. Also, you don't have to worry about switching data/settings between ROMs (they're shared), but you just need to regularly backup your important data (which is healthy anyway).
Click to expand...
Click to collapse
Can your elaborate where data is stored?
The userdata partition is also shared; so, you'll have access to all your FULL storage partitions & all apps/data similarly on either stock or virtual ROMs. This also solves the Safestrap issue of having less storage on ROM slots...
Click to expand...
Click to collapse
Will mSDcard incur a significant performance penalty on some devices?
there's no diffrerence between virtual & stock ROMs in terms of performance & battery life. The reason is simple: loop devices associated with the READ-ONLY system IMG mounted from EXT4 partition using a high-quality microSD card IS very fast more than enough.
The read speed is faster than the device can operate anyway + the exact same device should perform on the lowest speed when reading/writing from/to the FAT/FAT32/ExFAT extSD card (where you store your files or even move apps!!!) anyway, which is much slower than the read speed of a loop device mounted from EXT4 partition.
That's why data partition is shared for many reasons, including the poor READ/WRITE performance.
Click to expand...
Click to collapse
If virtual systems are read only, how do we modify them? Do we have to boot to another multisystem rom to modify a virtual rom?
The stock system partition is mounted by default read only & so are the virtual systems. To modify a stock/virtual system, the MultiSystem APK remounts them read/write. You can modify the currently running virtual system, copy it & modify the copy, modify another stock/virtual system.
Click to expand...
Click to collapse
How is a corrupted virtual rom handled? Does it see it's bad and default to stock system?
At early boot, MultiSystem checks for the microSD & active virtual ROM to boot it. There's a boot menu that gives you options to select a stock/virtual system, but it crashes on LP. I'm debugging it, but all functions won't be affected if I removed it. To fail safe, you can remove the microSD card to boot to stock system & restore/repair your virtual ROMs.
UPDATE1: MultiSystem v1.0.1 now allows you to also switch to stock system on boot to repair corrupted virtual IMGs or any other reasons. More options will be added during boot to ultimately select another virtual system if the active IMG is not booting normally (e.g., bootloop after applying a mod or flashing a bad .ZIP).
UPDATE2: Now, on boot, you can choose from two primary/secondary virtual ROM or stock ROM. Flashing multiple ROMs at the same time without a reboot is now possible.
Click to expand...
Click to collapse
How to check if an IMG is corrupted using MultiSystem status?
Code:
Current System IMG: Test_Rom.img
Current System DEV: [B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
When you see "/dev/block/mmcblk0p23"; it's the original system partition; so MultiSystem failed to boot Test_Rom.img, but it should be your current system.
So, the check is simple based on "Current System Device":
/dev/block/mmcblk0p23 = Stock System Partition
/dev/block/loop0 = Virtual System IMG
Note: The block device number (mmcblk0p23) may vary per device & per variant !
Click to expand...
Click to collapse
Does android do any maintenance whatsoever on stored data within /data or external sd? So if I have an app installed on 1 system and not on another system will android see it and clear the data?
No, all storage partitions are shared between ROMs. If you installed an app, it'll be availabe for all of them. Since on locked devcies we're limited to stock manufacturer-based ROMs, this makes the switch between ROMs very convinient (you don't have to worry about your changes/data/setup & storage space on the another ROM; all ROMs share everything except system). However, you should make regular backups in case a virtual ROM (probably with unsafe mods) results in bootloop due to your user data. In this case, it's safe to wipe data & selectively restore apps/data from backup(s). Another advantage of sharing all storage partitions is that your messages/emails/etc received on a virtual ROM are immediated synced (actually shared) to the other ROMs.
Click to expand...
Click to collapse
Will anything like Xposed modify the virtual ROM system IMG as opposed to the stock system IMG?
When you run a Virtual System, everything incldung kernel & apps are hijacked to speak to it as the original system.
Click to expand...
Click to collapse
Can we install AOSP ROMs on locked devices?
You can only install stock/manufacturer-based ROMs on locked devices while unlocked devices can use kexec or flash the required kernel to boot any AOSP/Stock ROMs. I've got a Note 4 Developer Edition & a lot of development is planned to go there (thanks to the unlocked bootloader!) More devices will get supported including unlocked TMO & international variants after adding more features untilizing the unlocked bootloader with kexec'd kernels.
Click to expand...
Click to collapse
Are there limitations to the combinations of ROMs that can be loaded on the "stock" and "virtual" slots? Can you mix KK and LP?
Yes, if they can run on the same kernel. LP won't run on KK kernels & so, you'd have to upgrade the firmware anyway. As for running mixed compatible Android versions, this is possible but your'd have to backup your data before switching ROMs; if it cause no issues, enjoy smooth switch & if it doesn't, do factory reset in recovery & restore your data backup. Backups via MultiSystem are painless.
Click to expand...
Click to collapse
Are applications installed once for each ROM slot that has that applicaiton installed, or can I share a game across ROMs (for instance?)
Everything is shared between ROMs, which is very good for storage & for easy switching. Just make regular backups of your sensitive data.
Click to expand...
Click to collapse
How there are no performance hits while internal storage memory was much faster than any microSD technology?
Read speeds from microSD is very fast compared to write speeds & since virtual ROMs are actually a virtual read-only systems (hence, MultiSystem), they provide a high performance. Moreover, again, read speeds from EXT4 loop devices are higher compared to physical partitions. They're very bad in writing, which we don't need for the read-only "system".
Click to expand...
Click to collapse
Is there a preferred "daily driver" ROM that should be installed in the stock slot?
Uses a stock ODEXED ROM on stock slot for better stability!
Click to expand...
Click to collapse
Is it based off of Safestrap?
Short answer NO. I've been working on MultiSystem & Safestrap for ~7 months. Earlier versions of MultiSystem (called, JasmineREC) was based on Safestrap, but it failed to support newer versions of Android mainly due to TWRP changes in the graphics/UI libraries that cause segmentation fault & the stock kernel framebuffer issues. Then, I decided to find another solution. However, the basic idea of system hijack is powered by Safestrap (or 2nd-init recoveries in general) & all the work done by @Hashcode is GREATLY appreciated.
Click to expand...
Click to collapse
How can it overwrite system files while running?
MultiSystem allows you to install safe mod's or a ROM in full or OTA-like update. It's strongly recommended to install .ZIP files NOT to the current system, b/c some files can not be overwritten while running. So, you can use backup function to copy the current system & install to the new img or any of your other virtual systems. You'll have several options to activate a virtual img & reboot directly to stock system, any virtual img you've activated, quick reboot, Download/bootloader, recovery,... etc.
Click to expand...
Click to collapse
How would I benefit from it if I'm only running Stock ROM or would there be no point for me to install it?
If you run a ROM on stock system, you're vulnerable to root loss unless/untill a new rooting method for LP comes out. MultiSystem gives you the option to run safe-to-mod virtual ROMs + recovery replacement + extra features.
Click to expand...
Click to collapse
Is there a way to convert a normal ROM .ZIP into MultiSystem .IMG?
Create or copy any of your IMGs, activate it & reboot to the active IMG! Then, use FlashFire to flash the ZIP file. However, the updater-script should be safe/compatible. Some devs mount the phyical partition, which will redirect everything to it!!
For example:
Code:
mount(“ext4″, “EMMC”, “/dev/block/mmcblk0p23″, “/system”);
will mount the original system partition; while
Code:
run_program("/sbin/mount", "-t", "auto", "/system");
will mount the current system (stock or virtual). This is recommended/safe.
Click to expand...
Click to collapse
Would a KitKat ROM work with multisystem even though my stock is Lollipop?
Any ROM requires a compatible kernel & modem. So, running KK ROMs requires flashing KK firmware (namely, kernel & modem). This may work with MultiSystem on other devices, especially if the bootlpoader is unlocked. For example, I plan to add features for Note 4 DevED to allow different Android versions (including AOSP, manufacturer-based, & probably Linux systems) by utilizing kernel swapping or execution.
Click to expand...
Click to collapse
When MultiSystem comes out will it be open sourced?
Most probably, haven't decided yet!
Anyway, here's the repository on GitHub: https://github.com/hsbadr/MultiSystem
Click to expand...
Click to collapse
Click to expand...
Click to collapse

Video Tutorials
A quick preview of MultiSystem v1.0 tested on Lollipop for VZW Note 3. The video has been captured on a stable virtual ROM of JasmineROM v5.0.1. It's FULLY compatible with FlashFire on virtual/stock systems. More devices will get supported as well, after required testing.
Facebook: https://www.facebook.com/hsbadr/videos/vb.331488823689599/428178174020663

How to check if you are running a Stock/Virtual System?
There're many ways to check whether you're running a Stock or Virtual system. MultiSystem app should include this simple check at some point. That's important to avoint ruining the Stock system & keep it safe. To make it clear to NOOBZ & anyone who's requesting "another" proof even though I owe hime nothing. Very weird!
Anyway, BusyBox mountpoint applet can print the current block/device mounted to /system mountpoint by running the following command:
Code:
busybox mountpoint -n /system
The stock system is mounts the original system partition:
Code:
[B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
while the virtual system mounts a loop device associated with a system IMG:
Code:
[B][COLOR="Blue"]/dev/block/loop0[/COLOR][/B]
Here're two videos for both stock & virtual systems...
UPDATE:
Now, you could run the following command to print the current system (stock or virtual) and the system device (physical partition or loop device):
Code:
MultiSystem status
Note: The block device number (mmcblk0p23) may vary per device & per variant !

How to repartition microSD card for MultiSystem?
You can use any tool/program for partitioning on Android, Linux, Mac, or Windows. For example, MiniTool Partition Wizard is a good partitioning tool for Windows. So, let's use it for this task. Simply, you need to follow this PDF tutorial (thanks to @carl1961). In sum:
Step 1: delete old partitions on SD card
Step 2: create FAT32 PRIMARY partition
Step 3: create EXT4 PRIMARY partition
Then, apply changes (note that the program UI may get changed in newer versions).
Notes:
This partitioning tutorial doesn't create PRIMARY partitions (it creates logical partitions). So, you need to change "Create As" from "Logical" to "Primary" when creatig a partition.
The sizes of the two partitions are arbitrary depending on number of ROMs you plan to install on the 2nd EXT4 partition.
The 1st partition (check size) is automatically detected as your external storage
In Terminal Emulator or ADB shell, check the existence of the two partitions by running the following command (in red):
Code:
[email protected]:/ # [COLOR="Red"]ls -l /dev/block/platform/msm_sdcc.3/[/COLOR]
drwxr-xr-x root root 2015-05-02 21:08 by-num
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1 -> /dev/block/mmcblk1
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p1 -> [COLOR="Blue"]/dev/block/mmcblk1p1[/COLOR]
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p2 -> [COLOR="Blue"]/dev/block/mmcblk1p2[/COLOR]
/dev/block/mmcblk1p1 is mounted by Android as your external storage.
/dev/block/mmcblk1p2 is NOT mounted & will be your MultiSystem partition.
Click to expand...
Click to collapse

How to check microSD card partitions for MultiSystem?
You need to correctly repartition microSD card into two partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
Use the directions in this post!
You should check your 2nd SD partition in EXT4 format mounted to /MultiSystem:
check that the /MultiSystem directory exists after a reboot
check that the 2nd SD partition (/dev/block/mmcblk1p2) is mounted to /MultiSystem by running the following command in Terminal Emulator or ADB shell:
Code:
mount | grep /MultiSystem
The output should be:
Code:
/dev/block/mmcblk1p2 /MultiSystem ext4 rw,seclabel,relatime,data=ordered 0 0

How to check MultiSystem Installation?
The 1st thing to do after installing MultiSystem is to check the /MultiSystem directory & its contents (it shouldn't be empty!). Then, check usage by running the following commands in Terminal Emulator or ADB shell:
Code:
su
bash
MultiSystem
If it retuns "MultiSystem not found" or permission denied, try to use open MultiSystem app to Update Configurations & try again. If this does't fix it, try the following command:
Code:
/MultiSystem/bin/MultiSystem
This should work if you've MultiSystem binaries installed in (extracted to) /MultiSystem directory. If so, you can create a symlink in /system/xbin as follows:
Code:
mount -o remount,rw /system
ln -sv /MultiSystem/bin/MultiSystem /system/xbin/MultiSystem
Then, test it by running:
Code:
MultiSystem
The last thing before using it is to check the boot options: reboot & monitor the GREEN LED indicator for 3 seconds (change in the app) , which give you the following options:
Volume UP = Primary virtual ROM
Volume DOWN = Secondary virtual ROM
HOME KEY = Stock System
Sure, you should have installed one or more virtual ROMs.
Backup & restore or creating/installing a virtual ROM are easy as copy & paste: all img's will be at
Code:
/MultiSystem/img/system
To backup a virtual/stock system, you have many options:
Use create function to create from stock system
Use copy function to copy the IMG
Copy & paste with a new name
Use FlashFire (fully supported on virtual/stock ROMs)
...
If you've IMG mounting issues, run the following commands:
Code:
mount -o remount,rw /system
busybox ln -sv /proc/self/mounts /system/etc/mtab
If this doesn't help, try mounting from Terminal Emulator or ADB shell after selecting the IMG in MultiSystem app, by running the following command:
Code:
MultiSystem mount virtual

Happy to see, one time a real describe of the features, all of my asks is answers here.
Thanks dev

Great work thanks

I've created 6GB partition for MultiSystem and everything until the green led is now working.
Now I'm on S6 port. If I were to flash Note4 port what do I need to do?. Can the current S6 port considered as stock?
I've read both threads but still not getting the procedures to install the virtual/secondary or any other rom.
Sorry for being a noob.

Rosli59564 said:
I've created 6GB partition for MultiSystem and everything until the green led is now working.
Now I'm on S6 port. If I were to flash Note4 port what do I need to do?. Can the current S6 port considered as stock?
I've read both threads but still not getting the procedures to install the virtual/secondary or any other rom.
Sorry for being a noob.
Click to expand...
Click to collapse
You can use any ROM as stock or virual, as far as your kernel supports them. Just take care when flashing; the update-script should mount the correct partition or IMG to /system. I'll explain this very soon...

MultiSystem Video Tutorial
Thanks To: @Tomsgt , aka RootJunky
Don't forget to subscribe & like the video to show appreciation of his great effort & time spent in making the video :highfive::good:

hsbadr said:
MultiSystem is a powerful tool for locked- and unlocked-bootloader Android devices with many features that at least includes the following:
Click to expand...
Click to collapse
My friend, this is EXTREMELY exciting, and I look forward to checking it out I just need some help:
I can't run the MiniTool Partition Manager on my computer because it isn't supported by the Windows 10 Tech Preview, so I wanted to ask if there was another way to create the other ext4 partition? Can it be done in Android? I am running Lollipop on the Canadian Note 3 model. Thank you, man
Sent from my SM-N900W8 using XDA Free mobile app

vince.lupe said:
My friend, this is EXTREMELY exciting, and I look forward to checking it out I just need some help:
I can't run the MiniTool Partition Manager on my computer because it isn't supported by the Windows 10 Tech Preview, so I wanted to ask if there was another way to create the other ext4 partition? Can it be done in Android? I am running Lollipop on the Canadian Note 3 model. Thank you, man
Click to expand...
Click to collapse
Aparted app from Play works, but not on LP (binaries needs to be compiled with PIE flag or you disable PIE check). Alternatively, you can find any partitioning too on your version of Windows that support EXT4 format OR better use MinTool Partition Manager on another computer or a VM on yours.

hsbadr said:
Aparted app from Play works, but not on LP (binaries needs to be compiled with PIE flag or you disable PIE check). Alternatively, you can find any partitioning too on your version of Windows that support EXT4 format OR better use MinTool Partition Manager on another computer or a VM on yours.
Click to expand...
Click to collapse
Ah, that's a great idea! I didn't even think of a VM! Would you mind suggesting the best way to create a VM of that program? Which VM program should I use? Also, how did you get Linux to run on your MultiSystem??? That would be WONDERFUL for me so that I could use Linux to run Microsoft SilverLight for university to watch videos (in the next few months, I want to migrate from Microsoft to Android completely)
Sent from my SM-N900W8 using XDA Free mobile app

Great work thanks

hsbadr said:
Aparted app from Play works, but not on LP (binaries needs to be compiled with PIE flag or you disable PIE check). Alternatively, you can find any partitioning too on your version of Windows that support EXT4 format OR better use MinTool Partition Manager on another computer or a VM on yours.
Click to expand...
Click to collapse
My friend, I ended up getting everything partitioned perfectly fine and have just started to look around I downloaded a fresh image for Linux from the Complete Linux Installer website, but I'm not sure if it's the right image for flashing with MultiSystem. Do you think you could give a few pointers? Thank you, man This is exciting
Sent from my SM-N900W8 using XDA Free mobile app

vince.lupe said:
My friend, I ended up getting everything partitioned perfectly fine and have just started to look around I downloaded a fresh image for Linux from the Complete Linux Installer website, but I'm not sure if it's the right image for flashing with MultiSystem. Do you think you could give a few pointers? Thank you, man This is exciting
Click to expand...
Click to collapse
You can't boot a Linux IMG in the current version of MultiSystem, b/c this feature is temporarily removed to avoid limited support for locked devices. However, you can CHROOT this IMG on Android & use any VNC app to connect to your Linux desktop on device. I'd recommend Linux Deploy.

hsbadr said:
You can't boot a Linux IMG in the current version of MultiSystem, b/c this feature is temporarily removed to avoid limited support for locked devices. However, you can CHROOT this IMG on Android & use any VNC app to connect to your Linux desktop on device. I'd recommend Linux Deploy.
Click to expand...
Click to collapse
That's cool, man Will you be including that feature in the next update?
Also, will Linux Deploy do all the work, or is there a way to get a Linux IMG to work with MultiSystem (understanding that, at present, some sort of work would be needed done)?
Thank you This is wonderful
Sent from my SM-N900W8 using XDA Free mobile app

vince.lupe said:
That's cool, man Will you be including that feature in the next update?
Click to expand...
Click to collapse
Not in the next version, but in device-specific updates (since each unlocked device should get a supported kernel 1st).
vince.lupe said:
Also, will Linux Deploy do all the work, or is there a way to get a Linux IMG to work with MultiSystem (understanding that, at present, some sort of work would be needed done)?
Click to expand...
Click to collapse
Linux Deploy will allow you to run Linux virtually on top of Android system. Then, you can connect to it via VNC...
vince.lupe said:
Thank you This is wonderful
Click to expand...
Click to collapse
You're welcome! Glad I could help.

hsbadr said:
Not in the next version, but in device-specific updates (since each unlocked device should get a supported kernel 1st).
Linux Deploy will allow you to run Linux virtually on top of Android system. Then, you can connect to it via VNC...
You're welcome! Glad I could help.
Click to expand...
Click to collapse
Looking forward for device specific updates for the Canadian Note 3 Your work is amazing Keep it up, bro
Sent from my SM-N900W8 using XDA Free mobile app

[Q&A] MultiSystem for Android

MultiSystem is a powerful tool for locked- and unlocked-bootloader Android devices with many features that at least includes the following:
Keeps stock system partition safe/rooted
Permenant root survival with proper use
MultiROM support via virtual ROMs
Unlimited number of virtual ROMs
Booting options to choose stock, primary, or secondary virtual ROM
Any of the virtual ROMs can work as a recovery replacement
Flashing multiple ROMs at the same time without a reboot
Ability to create/install ROMs on Linux to microSD card
Great performance & battery life on virtual ROMs
Recovery solution to install ROMs or Mods
Easy upgrade to newer versions of Android
Ability to safely apply OTA updates to virtual system
Permissive SELinux and other kernel tweaks
Safe flashing that doesn't trip KNOX flag on Samsung devices
Wrapper script runs via ADB or a Terminal Emulator on device
APK to manage all MultiSystem functions with a nice UI and extra options
Management for the best performance & user experience
Support for all Android devices with microSD card
Portability to almost all devices
Compatibility with all Android versions
Click to expand...
Click to collapse
Q&A​
What is the concept behind MultiSystem?
It runs virtual Android ROMs on microSD, like booting multiple systems on a PC from different partitions/disks. So, your stock system partition is kept safe/rooted. It won't affect performance or anything (might even be better on the virtual system if you've high quality microSD & the device supports its speed). Also, you can freely modify any of the virtual systems & in the worst case, reboot the safe stock system or another working virtual system to recover. So, no root loss or potential damage to the original device partitions.
Click to expand...
Click to collapse
Is it a recovery or an APK tool?
It's a shell script that hijacks system at early boot & force Android to boot from the stock system partition or a virtual system IMG & an APK that manages all booting options, virtual ROMs, and works as a recovery replacement + extra features...
Click to expand...
Click to collapse
Does it work as a recovery replacement?
It IS a POWERFUL recovery replacement. You can do whatever you do in recovery with the APK. HOW? recovery does its magic b/c it doesn't depend on the system & has its own kernel/ramdisk. In MultiSystem, you can boot a virtual ROM from extSD that sure doesn't depend on stock system partition or any of the other virtual ROMs (it does depend on the kernel, which you can't flash on locked devcies anyway). Hence, install, backup, restore, ... & all recovery functions are all possible +++ more features since you're running a full ROM not just a recovery ramdisk like Safestrap.
Bottom Line: I think it's the best & most convenient recovery replacement ever for locked devices & it can also attract unlocked devices for the powerful features, MultiROM, and recovery from within ROM.
Click to expand...
Click to collapse
Can I use FlashFire along with MultiSystem?
Yes. MultiSystem is compatible with FlashFire & fully supports it on stock & virtual ROMs. So, you can use both/any of them for flashing to either a stock or virtual ROM. However, it's recommended to use MultiSystem when flashing to the stock system partition (shouldn't be needed anyway since you can always be safe & flash to your old/new virtual ROMs).
Click to expand...
Click to collapse
Does MultiSystem require FlashFire?
No, MultiSystem doesn't require FlashFire. They're fully combatible though.
Click to expand...
Click to collapse
Would the virtual ROM we install be exactly the one in the stock slot?
In MultiSystem APK, you can create a virtual ROM from stock system, a copy from other virtual ROM, a new IMG, a dev-provided ROM, a flashable .ZIP, ... etc. Literally, your virtual ROMs can be any stock or custom ROM that's compatible with your firmware/kernel.
Click to expand...
Click to collapse
How can it run virtual ROMs from external microSD card?
External MicroSD will be formated into 2 partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
It'll hijack the system & boot a virtual system from the 2nd partition. The 1st partition will be automatically detected as your extSD.
Click to expand...
Click to collapse
Can I run unrooted virtual ROM for work apps or any other reason?
Yes. You can add unrooted virtual ROM & reboot to it via MultiSystem APK.
Click to expand...
Click to collapse
How do you boot back into a different ROM?
MultiSystem APK manages all functions including ROM activation & reboot to current system, another stock/virtual system, download mode, recovery, ... etc.
Click to expand...
Click to collapse
Will it be OK to still store media like movies/photos/music to extSD?
100% OK; That's my setup a few months ago. 2 virtual ROMs in the SECOND extSD partition in EXT4 format while all personal data are stored on the FIRST extSD partition in exFAT or FAT32 format... TWO COMPLETELY DIFFERET PARTITIONS.
Click to expand...
Click to collapse
How much space are we going to have for virtual ROMs?
The size of the 2nd partition is optional (> 4GB) for your ROMs, but here is an estimated sizes:
1 Virtual ROM Uncompressed = ~2.7 GB ---> ready for running
1 Virtual ROM Compressed = ~1.5 GB ---> for full ROM backups
I'd say better allocate 4 GB for each ROM you plan to run. If you just need one virtual ROM to keep stock system safe, 4 GB 2nd extSD partition is enough; The remaining space is allocated for the 1st extSD partition as your external storage.
For me, I run Linux too from extSD via MultiSystem. So, I've 64 GB extSD card with two partitions 32 GB each.
Click to expand...
Click to collapse
Can I clear up space on an existing SD card and partition it while full or will the entire card need to be wiped and partitioned from scratch?
You need to backup all your files; it'll be wiped & repartitioned.
Click to expand...
Click to collapse
How can I swap microSD cards & be able to run virtual ROMs?
You can swap microSD cards as you wish provided that the device is powered off; don't remove the microSD card when running a virtual ROM. If the new microSD card doesn't include a 2nd parition of available virtual ROMs, the device will boot directly to the stock system.
Click to expand...
Click to collapse
Is there a specific sd card you recommended for this?
I personally have two microSD cards:
SanDisk Extreme Plus 64GB (Up to 80MB/s read speed)
Samsung 64GB PRO (Up to 90MB/s read speed)
You don't have to change your microSD card for MultiSystem; any card you use on your device should work just fine. The need for more speed is relevant when the device supports that speed & if you're going to buy a new card anyway that you may use with a newer device later.
Click to expand...
Click to collapse
Can I copy virtual ROMs to a new microSD card?
Yes. I'll add a feature for swapping microSD cards so that you can backup/restore virtual ROMs from/to the current extSD to/from internal storage as follows:
power off device
use MultiSystem APK to backup your virtual ROMs
insert the new properly formatted microSD,
power on device (it'll boot to stock system)
use MultiSystem APK to restore your virtual ROMs
use MultiSystem APK to activate one of your virtual ROMs
use MultiSystem APK to reboot to any of your ROMs
Click to expand...
Click to collapse
What about other data/cache partitions and internal storage?
Only system img's are in the extSD. All ROMs share all other partitions. This substantially improves the performance & you won't notice any difference between your stock & virtual ROMs. The reason for performance improvement is that EXT4 loop devices are very fast in reading but not in writing. Your system partition is read-only while data (for example) is read write & cache IMGs cause problems like Safestrap issues on ROM slots. Also, you don't have to worry about switching data/settings between ROMs (they're shared), but you just need to regularly backup your important data (which is healthy anyway).
Click to expand...
Click to collapse
Can your elaborate where data is stored?
The userdata partition is also shared; so, you'll have access to all your FULL storage partitions & all apps/data similarly on either stock or virtual ROMs. This also solves the Safestrap issue of having less storage on ROM slots...
Click to expand...
Click to collapse
Will mSDcard incur a significant performance penalty on some devices?
there's no diffrerence between virtual & stock ROMs in terms of performance & battery life. The reason is simple: loop devices associated with the READ-ONLY system IMG mounted from EXT4 partition using a high-quality microSD card IS very fast more than enough.
The read speed is faster than the device can operate anyway + the exact same device should perform on the lowest speed when reading/writing from/to the FAT/FAT32/ExFAT extSD card (where you store your files or even move apps!!!) anyway, which is much slower than the read speed of a loop device mounted from EXT4 partition.
That's why data partition is shared for many reasons, including the poor READ/WRITE performance.
Click to expand...
Click to collapse
If virtual systems are read only, how do we modify them? Do we have to boot to another multisystem rom to modify a virtual rom?
The stock system partition is mounted by default read only & so are the virtual systems. To modify a stock/virtual system, the MultiSystem APK remounts them read/write. You can modify the currently running virtual system, copy it & modify the copy, modify another stock/virtual system.
Click to expand...
Click to collapse
How is a corrupted virtual rom handled? Does it see it's bad and default to stock system?
At early boot, MultiSystem checks for the microSD & active virtual ROM to boot it. There's a boot menu that gives you options to select a stock/virtual system, but it crashes on LP. I'm debugging it, but all functions won't be affected if I removed it. To fail safe, you can remove the microSD card to boot to stock system & restore/repair your virtual ROMs.
UPDATE1: MultiSystem v1.0.1 now allows you to also switch to stock system on boot to repair corrupted virtual IMGs or any other reasons. More options will be added during boot to ultimately select another virtual system if the active IMG is not booting normally (e.g., bootloop after applying a mod or flashing a bad .ZIP).
UPDATE2: Now, on boot, you can choose from two primary/secondary virtual ROM or stock ROM. Flashing multiple ROMs at the same time without a reboot is now possible.
Click to expand...
Click to collapse
How to check if an IMG is corrupted using MultiSystem status?
Code:
Current System IMG: Test_Rom.img
Current System DEV: [B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
When you see "/dev/block/mmcblk0p23"; it's the original system partition; so MultiSystem failed to boot Test_Rom.img, but it should be your current system.
So, the check is simple based on "Current System Device":
/dev/block/mmcblk0p23 = Stock System Partition
/dev/block/loop0 = Virtual System IMG
Note: The block device number (mmcblk0p23) may vary per device & per variant !
Click to expand...
Click to collapse
Does android do any maintenance whatsoever on stored data within /data or external sd? So if I have an app installed on 1 system and not on another system will android see it and clear the data?
No, all storage partitions are shared between ROMs. If you installed an app, it'll be availabe for all of them. Since on locked devcies we're limited to stock manufacturer-based ROMs, this makes the switch between ROMs very convinient (you don't have to worry about your changes/data/setup & storage space on the another ROM; all ROMs share everything except system). However, you should make regular backups in case a virtual ROM (probably with unsafe mods) results in bootloop due to your user data. In this case, it's safe to wipe data & selectively restore apps/data from backup(s). Another advantage of sharing all storage partitions is that your messages/emails/etc received on a virtual ROM are immediated synced (actually shared) to the other ROMs.
Click to expand...
Click to collapse
Will anything like Xposed modify the virtual ROM system IMG as opposed to the stock system IMG?
When you run a Virtual System, everything incldung kernel & apps are hijacked to speak to it as the original system.
Click to expand...
Click to collapse
Can we install AOSP ROMs on locked devices?
You can only install stock/manufacturer-based ROMs on locked devices while unlocked devices can use kexec or flash the required kernel to boot any AOSP/Stock ROMs. I've got a Note 4 Developer Edition & a lot of development is planned to go there (thanks to the unlocked bootloader!) More devices will get supported including unlocked TMO & international variants after adding more features untilizing the unlocked bootloader with kexec'd kernels.
Click to expand...
Click to collapse
Are there limitations to the combinations of ROMs that can be loaded on the "stock" and "virtual" slots? Can you mix KK and LP?
Yes, if they can run on the same kernel. LP won't run on KK kernels & so, you'd have to upgrade the firmware anyway. As for running mixed compatible Android versions, this is possible but your'd have to backup your data before switching ROMs; if it cause no issues, enjoy smooth switch & if it doesn't, do factory reset in recovery & restore your data backup. Backups via MultiSystem are painless.
Click to expand...
Click to collapse
Are applications installed once for each ROM slot that has that applicaiton installed, or can I share a game across ROMs (for instance?)
Everything is shared between ROMs, which is very good for storage & for easy switching. Just make regular backups of your sensitive data.
Click to expand...
Click to collapse
How there are no performance hits while internal storage memory was much faster than any microSD technology?
Read speeds from microSD is very fast compared to write speeds & since virtual ROMs are actually a virtual read-only systems (hence, MultiSystem), they provide a high performance. Moreover, again, read speeds from EXT4 loop devices are higher compared to physical partitions. They're very bad in writing, which we don't need for the read-only "system".
Click to expand...
Click to collapse
Is there a preferred "daily driver" ROM that should be installed in the stock slot?
Uses a stock ODEXED ROM on stock slot for better stability!
Click to expand...
Click to collapse
Is it based off of Safestrap?
Short answer NO. I've been working on MultiSystem & Safestrap for ~7 months. Earlier versions of MultiSystem (called, JasmineREC) was based on Safestrap, but it failed to support newer versions of Android mainly due to TWRP changes in the graphics/UI libraries that cause segmentation fault & the stock kernel framebuffer issues. Then, I decided to find another solution. However, the basic idea of system hijack is powered by Safestrap (or 2nd-init recoveries in general) & all the work done by @Hashcode is GREATLY appreciated.
Click to expand...
Click to collapse
How can it overwrite system files while running?
MultiSystem allows you to install safe mod's or a ROM in full or OTA-like update. It's strongly recommended to install .ZIP files NOT to the current system, b/c some files can not be overwritten while running. So, you can use backup function to copy the current system & install to the new img or any of your other virtual systems. You'll have several options to activate a virtual img & reboot directly to stock system, any virtual img you've activated, quick reboot, Download/bootloader, recovery,... etc.
Click to expand...
Click to collapse
How would I benefit from it if I'm only running Stock ROM or would there be no point for me to install it?
If you run a ROM on stock system, you're vulnerable to root loss unless/untill a new rooting method for LP comes out. MultiSystem gives you the option to run safe-to-mod virtual ROMs + recovery replacement + extra features.
Click to expand...
Click to collapse
Is there a way to convert a normal ROM .ZIP into MultiSystem .IMG?
Create or copy any of your IMGs, activate it & reboot to the active IMG! Then, use FlashFire to flash the ZIP file. However, the updater-script should be safe/compatible. Some devs mount the phyical partition, which will redirect everything to it!!
For example:
Code:
mount(“ext4″, “EMMC”, “/dev/block/mmcblk0p23″, “/system”);
will mount the original system partition; while
Code:
run_program("/sbin/mount", "-t", "auto", "/system");
will mount the current system (stock or virtual). This is recommended/safe.
Click to expand...
Click to collapse
Would a KitKat ROM work with multisystem even though my stock is Lollipop?
Any ROM requires a compatible kernel & modem. So, running KK ROMs requires flashing KK firmware (namely, kernel & modem). This may work with MultiSystem on other devices, especially if the bootlpoader is unlocked. For example, I plan to add features for Note 4 DevED to allow different Android versions (including AOSP, manufacturer-based, & probably Linux systems) by utilizing kernel swapping or execution.
Click to expand...
Click to collapse
When MultiSystem comes out will it be open sourced?
Most probably, haven't decided yet!
Anyway, here's the repository on GitHub: https://github.com/hsbadr/MultiSystem
Click to expand...
Click to collapse
Click to expand...
Click to collapse

Video Tutorials
A quick preview of MultiSystem v1.0 tested on Lollipop for VZW Note 3. The video has been captured on a stable virtual ROM of JasmineROM v5.0.1. It's FULLY compatible with FlashFire on virtual/stock systems. More devices will get supported as well, after required testing.
Facebook: https://www.facebook.com/hsbadr/videos/vb.331488823689599/428178174020663

How to check if you are running a Stock/Virtual System?
There're many ways to check whether you're running a Stock or Virtual system. MultiSystem app should include this simple check at some point. That's important to avoint ruining the Stock system & keep it safe. To make it clear to NOOBZ & anyone who's requesting "another" proof even though I owe hime nothing. Very weird!
Anyway, BusyBox mountpoint applet can print the current block/device mounted to /system mountpoint by running the following command:
Code:
busybox mountpoint -n /system
The stock system is mounts the original system partition:
Code:
[B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
while the virtual system mounts a loop device associated with a system IMG:
Code:
[B][COLOR="Blue"]/dev/block/loop0[/COLOR][/B]
Here're two videos for both stock & virtual systems...
UPDATE:
Now, you could run the following command to print the current system (stock or virtual) and the system device (physical partition or loop device):
Code:
MultiSystem status
Note: The block device number (mmcblk0p23) may vary per device & per variant !

How to repartition microSD card for MultiSystem?
You can use any tool/program for partitioning on Android, Linux, Mac, or Windows. For example, MiniTool Partition Wizard is a good partitioning tool for Windows. So, let's use it for this task. Simply, you need to follow this PDF tutorial (thanks to @carl1961). In sum:
Step 1: delete old partitions on SD card
Step 2: create FAT32 PRIMARY partition
Step 3: create EXT4 PRIMARY partition
Then, apply changes (note that the program UI may get changed in newer versions).
Notes:
This partitioning tutorial doesn't create PRIMARY partitions (it creates logical partitions). So, you need to change "Create As" from "Logical" to "Primary" when creatig a partition.
The sizes of the two partitions are arbitrary depending on number of ROMs you plan to install on the 2nd EXT4 partition.
The 1st partition (check size) is automatically detected as your external storage
In Terminal Emulator or ADB shell, check the existence of the two partitions by running the following command (in red):
Code:
[email protected]:/ # [COLOR="Red"]ls -l /dev/block/platform/msm_sdcc.3/[/COLOR]
drwxr-xr-x root root 2015-05-02 21:08 by-num
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1 -> /dev/block/mmcblk1
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p1 -> [COLOR="Blue"]/dev/block/mmcblk1p1[/COLOR]
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p2 -> [COLOR="Blue"]/dev/block/mmcblk1p2[/COLOR]
/dev/block/mmcblk1p1 is mounted by Android as your external storage.
/dev/block/mmcblk1p2 is NOT mounted & will be your MultiSystem partition.
Click to expand...
Click to collapse

How to check microSD card partitions for MultiSystem?
You need to correctly repartition microSD card into two partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
Use the directions in this post!
You should check your 2nd SD partition in EXT4 format mounted to /MultiSystem:
check that the /MultiSystem directory exists after a reboot
check that the 2nd SD partition (/dev/block/mmcblk1p2) is mounted to /MultiSystem by running the following command in Terminal Emulator or ADB shell:
Code:
mount | grep /MultiSystem
The output should be:
Code:
/dev/block/mmcblk1p2 /MultiSystem ext4 rw,seclabel,relatime,data=ordered 0 0

How to check MultiSystem Installation?
The 1st thing to do after installing MultiSystem is to check the /MultiSystem directory & its contents (it shouldn't be empty!). Then, check usage by running the following commands in Terminal Emulator or ADB shell:
Code:
su
bash
MultiSystem
If it retuns "MultiSystem not found" or permission denied, try to use open MultiSystem app to Update Configurations & try again. If this does't fix it, try the following command:
Code:
/MultiSystem/bin/MultiSystem
This should work if you've MultiSystem binaries installed in (extracted to) /MultiSystem directory. If so, you can create a symlink in /system/xbin as follows:
Code:
mount -o remount,rw /system
ln -sv /MultiSystem/bin/MultiSystem /system/xbin/MultiSystem
Then, test it by running:
Code:
MultiSystem
The last thing before using it is to check the boot options: reboot & monitor the GREEN LED indicator for 3 seconds (change in the app) , which give you the following options:
Volume UP = Primary virtual ROM
Volume DOWN = Secondary virtual ROM
HOME KEY = Stock System
Sure, you should have installed one or more virtual ROMs.
Backup & restore or creating/installing a virtual ROM are easy as copy & paste: all img's will be at
Code:
/MultiSystem/img/system
To backup a virtual/stock system, you have many options:
Use create function to create from stock system
Use copy function to copy the IMG
Copy & paste with a new name
Use FlashFire (fully supported on virtual/stock ROMs)
...
If you've IMG mounting issues, run the following commands:
Code:
mount -o remount,rw /system
busybox ln -sv /proc/self/mounts /system/etc/mtab
If this doesn't help, try mounting from Terminal Emulator or ADB shell after selecting the IMG in MultiSystem app, by running the following command:
Code:
MultiSystem mount virtual

MultiSystem Video Tutorial
Thanks To: @Tomsgt , aka RootJunky
Don't forget to subscribe & like the video to show appreciation of his great effort & time spent in making the video :highfive::good:

[Q&A] MultiSystem for Android

MultiSystem is a powerful tool for locked- and unlocked-bootloader Android devices with many features that at least includes the following:
Keeps stock system partition safe/rooted
Permenant root survival with proper use
MultiROM support via virtual ROMs
Unlimited number of virtual ROMs
Booting options to choose stock, primary, or secondary virtual ROM
Any of the virtual ROMs can work as a recovery replacement
Flashing multiple ROMs at the same time without a reboot
Ability to create/install ROMs on Linux to microSD card
Great performance & battery life on virtual ROMs
Recovery solution to install ROMs or Mods
Easy upgrade to newer versions of Android
Ability to safely apply OTA updates to virtual system
Permissive SELinux and other kernel tweaks
Safe flashing that doesn't trip KNOX flag on Samsung devices
Wrapper script runs via ADB or a Terminal Emulator on device
APK to manage all MultiSystem functions with a nice UI and extra options
Management for the best performance & user experience
Support for all Android devices with microSD card
Portability to almost all devices
Compatibility with all Android versions
Click to expand...
Click to collapse
Q&A​
What is the concept behind MultiSystem?
It runs virtual Android ROMs on microSD, like booting multiple systems on a PC from different partitions/disks. So, your stock system partition is kept safe/rooted. It won't affect performance or anything (might even be better on the virtual system if you've high quality microSD & the device supports its speed). Also, you can freely modify any of the virtual systems & in the worst case, reboot the safe stock system or another working virtual system to recover. So, no root loss or potential damage to the original device partitions.
Click to expand...
Click to collapse
Is it a recovery or an APK tool?
It's a shell script that hijacks system at early boot & force Android to boot from the stock system partition or a virtual system IMG & an APK that manages all booting options, virtual ROMs, and works as a recovery replacement + extra features...
Click to expand...
Click to collapse
Does it work as a recovery replacement?
It IS a POWERFUL recovery replacement. You can do whatever you do in recovery with the APK. HOW? recovery does its magic b/c it doesn't depend on the system & has its own kernel/ramdisk. In MultiSystem, you can boot a virtual ROM from extSD that sure doesn't depend on stock system partition or any of the other virtual ROMs (it does depend on the kernel, which you can't flash on locked devcies anyway). Hence, install, backup, restore, ... & all recovery functions are all possible +++ more features since you're running a full ROM not just a recovery ramdisk like Safestrap.
Bottom Line: I think it's the best & most convenient recovery replacement ever for locked devices & it can also attract unlocked devices for the powerful features, MultiROM, and recovery from within ROM.
Click to expand...
Click to collapse
Can I use FlashFire along with MultiSystem?
Yes. MultiSystem is compatible with FlashFire & fully supports it on stock & virtual ROMs. So, you can use both/any of them for flashing to either a stock or virtual ROM. However, it's recommended to use MultiSystem when flashing to the stock system partition (shouldn't be needed anyway since you can always be safe & flash to your old/new virtual ROMs).
Click to expand...
Click to collapse
Does MultiSystem require FlashFire?
No, MultiSystem doesn't require FlashFire. They're fully combatible though.
Click to expand...
Click to collapse
Would the virtual ROM we install be exactly the one in the stock slot?
In MultiSystem APK, you can create a virtual ROM from stock system, a copy from other virtual ROM, a new IMG, a dev-provided ROM, a flashable .ZIP, ... etc. Literally, your virtual ROMs can be any stock or custom ROM that's compatible with your firmware/kernel.
Click to expand...
Click to collapse
How can it run virtual ROMs from external microSD card?
External MicroSD will be formated into 2 partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
It'll hijack the system & boot a virtual system from the 2nd partition. The 1st partition will be automatically detected as your extSD.
Click to expand...
Click to collapse
Can I run unrooted virtual ROM for work apps or any other reason?
Yes. You can add unrooted virtual ROM & reboot to it via MultiSystem APK.
Click to expand...
Click to collapse
How do you boot back into a different ROM?
MultiSystem APK manages all functions including ROM activation & reboot to current system, another stock/virtual system, download mode, recovery, ... etc.
Click to expand...
Click to collapse
Will it be OK to still store media like movies/photos/music to extSD?
100% OK; That's my setup a few months ago. 2 virtual ROMs in the SECOND extSD partition in EXT4 format while all personal data are stored on the FIRST extSD partition in exFAT or FAT32 format... TWO COMPLETELY DIFFERET PARTITIONS.
Click to expand...
Click to collapse
How much space are we going to have for virtual ROMs?
The size of the 2nd partition is optional (> 4GB) for your ROMs, but here is an estimated sizes:
1 Virtual ROM Uncompressed = ~2.7 GB ---> ready for running
1 Virtual ROM Compressed = ~1.5 GB ---> for full ROM backups
I'd say better allocate 4 GB for each ROM you plan to run. If you just need one virtual ROM to keep stock system safe, 4 GB 2nd extSD partition is enough; The remaining space is allocated for the 1st extSD partition as your external storage.
For me, I run Linux too from extSD via MultiSystem. So, I've 64 GB extSD card with two partitions 32 GB each.
Click to expand...
Click to collapse
Can I clear up space on an existing SD card and partition it while full or will the entire card need to be wiped and partitioned from scratch?
You need to backup all your files; it'll be wiped & repartitioned.
Click to expand...
Click to collapse
How can I swap microSD cards & be able to run virtual ROMs?
You can swap microSD cards as you wish provided that the device is powered off; don't remove the microSD card when running a virtual ROM. If the new microSD card doesn't include a 2nd parition of available virtual ROMs, the device will boot directly to the stock system.
Click to expand...
Click to collapse
Is there a specific sd card you recommended for this?
I personally have two microSD cards:
SanDisk Extreme Plus 64GB (Up to 80MB/s read speed)
Samsung 64GB PRO (Up to 90MB/s read speed)
You don't have to change your microSD card for MultiSystem; any card you use on your device should work just fine. The need for more speed is relevant when the device supports that speed & if you're going to buy a new card anyway that you may use with a newer device later.
Click to expand...
Click to collapse
Can I copy virtual ROMs to a new microSD card?
Yes. I'll add a feature for swapping microSD cards so that you can backup/restore virtual ROMs from/to the current extSD to/from internal storage as follows:
power off device
use MultiSystem APK to backup your virtual ROMs
insert the new properly formatted microSD,
power on device (it'll boot to stock system)
use MultiSystem APK to restore your virtual ROMs
use MultiSystem APK to activate one of your virtual ROMs
use MultiSystem APK to reboot to any of your ROMs
Click to expand...
Click to collapse
What about other data/cache partitions and internal storage?
Only system img's are in the extSD. All ROMs share all other partitions. This substantially improves the performance & you won't notice any difference between your stock & virtual ROMs. The reason for performance improvement is that EXT4 loop devices are very fast in reading but not in writing. Your system partition is read-only while data (for example) is read write & cache IMGs cause problems like Safestrap issues on ROM slots. Also, you don't have to worry about switching data/settings between ROMs (they're shared), but you just need to regularly backup your important data (which is healthy anyway).
Click to expand...
Click to collapse
Can your elaborate where data is stored?
The userdata partition is also shared; so, you'll have access to all your FULL storage partitions & all apps/data similarly on either stock or virtual ROMs. This also solves the Safestrap issue of having less storage on ROM slots...
Click to expand...
Click to collapse
Will mSDcard incur a significant performance penalty on some devices?
there's no diffrerence between virtual & stock ROMs in terms of performance & battery life. The reason is simple: loop devices associated with the READ-ONLY system IMG mounted from EXT4 partition using a high-quality microSD card IS very fast more than enough.
The read speed is faster than the device can operate anyway + the exact same device should perform on the lowest speed when reading/writing from/to the FAT/FAT32/ExFAT extSD card (where you store your files or even move apps!!!) anyway, which is much slower than the read speed of a loop device mounted from EXT4 partition.
That's why data partition is shared for many reasons, including the poor READ/WRITE performance.
Click to expand...
Click to collapse
If virtual systems are read only, how do we modify them? Do we have to boot to another multisystem rom to modify a virtual rom?
The stock system partition is mounted by default read only & so are the virtual systems. To modify a stock/virtual system, the MultiSystem APK remounts them read/write. You can modify the currently running virtual system, copy it & modify the copy, modify another stock/virtual system.
Click to expand...
Click to collapse
How is a corrupted virtual rom handled? Does it see it's bad and default to stock system?
At early boot, MultiSystem checks for the microSD & active virtual ROM to boot it. There's a boot menu that gives you options to select a stock/virtual system, but it crashes on LP. I'm debugging it, but all functions won't be affected if I removed it. To fail safe, you can remove the microSD card to boot to stock system & restore/repair your virtual ROMs.
UPDATE1: MultiSystem v1.0.1 now allows you to also switch to stock system on boot to repair corrupted virtual IMGs or any other reasons. More options will be added during boot to ultimately select another virtual system if the active IMG is not booting normally (e.g., bootloop after applying a mod or flashing a bad .ZIP).
UPDATE2: Now, on boot, you can choose from two primary/secondary virtual ROM or stock ROM. Flashing multiple ROMs at the same time without a reboot is now possible.
Click to expand...
Click to collapse
How to check if an IMG is corrupted using MultiSystem status?
Code:
Current System IMG: Test_Rom.img
Current System DEV: [B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
When you see "/dev/block/mmcblk0p23"; it's the original system partition; so MultiSystem failed to boot Test_Rom.img, but it should be your current system.
So, the check is simple based on "Current System Device":
/dev/block/mmcblk0p23 = Stock System Partition
/dev/block/loop0 = Virtual System IMG
Note: The block device number (mmcblk0p23) may vary per device & per variant !
Click to expand...
Click to collapse
Does android do any maintenance whatsoever on stored data within /data or external sd? So if I have an app installed on 1 system and not on another system will android see it and clear the data?
No, all storage partitions are shared between ROMs. If you installed an app, it'll be availabe for all of them. Since on locked devcies we're limited to stock manufacturer-based ROMs, this makes the switch between ROMs very convinient (you don't have to worry about your changes/data/setup & storage space on the another ROM; all ROMs share everything except system). However, you should make regular backups in case a virtual ROM (probably with unsafe mods) results in bootloop due to your user data. In this case, it's safe to wipe data & selectively restore apps/data from backup(s). Another advantage of sharing all storage partitions is that your messages/emails/etc received on a virtual ROM are immediated synced (actually shared) to the other ROMs.
Click to expand...
Click to collapse
Will anything like Xposed modify the virtual ROM system IMG as opposed to the stock system IMG?
When you run a Virtual System, everything incldung kernel & apps are hijacked to speak to it as the original system.
Click to expand...
Click to collapse
Can we install AOSP ROMs on locked devices?
You can only install stock/manufacturer-based ROMs on locked devices while unlocked devices can use kexec or flash the required kernel to boot any AOSP/Stock ROMs. I've got a Note 4 Developer Edition & a lot of development is planned to go there (thanks to the unlocked bootloader!) More devices will get supported including unlocked TMO & international variants after adding more features untilizing the unlocked bootloader with kexec'd kernels.
Click to expand...
Click to collapse
Are there limitations to the combinations of ROMs that can be loaded on the "stock" and "virtual" slots? Can you mix KK and LP?
Yes, if they can run on the same kernel. LP won't run on KK kernels & so, you'd have to upgrade the firmware anyway. As for running mixed compatible Android versions, this is possible but your'd have to backup your data before switching ROMs; if it cause no issues, enjoy smooth switch & if it doesn't, do factory reset in recovery & restore your data backup. Backups via MultiSystem are painless.
Click to expand...
Click to collapse
Are applications installed once for each ROM slot that has that applicaiton installed, or can I share a game across ROMs (for instance?)
Everything is shared between ROMs, which is very good for storage & for easy switching. Just make regular backups of your sensitive data.
Click to expand...
Click to collapse
How there are no performance hits while internal storage memory was much faster than any microSD technology?
Read speeds from microSD is very fast compared to write speeds & since virtual ROMs are actually a virtual read-only systems (hence, MultiSystem), they provide a high performance. Moreover, again, read speeds from EXT4 loop devices are higher compared to physical partitions. They're very bad in writing, which we don't need for the read-only "system".
Click to expand...
Click to collapse
Is there a preferred "daily driver" ROM that should be installed in the stock slot?
Uses a stock ODEXED ROM on stock slot for better stability!
Click to expand...
Click to collapse
Is it based off of Safestrap?
Short answer NO. I've been working on MultiSystem & Safestrap for ~7 months. Earlier versions of MultiSystem (called, JasmineREC) was based on Safestrap, but it failed to support newer versions of Android mainly due to TWRP changes in the graphics/UI libraries that cause segmentation fault & the stock kernel framebuffer issues. Then, I decided to find another solution. However, the basic idea of system hijack is powered by Safestrap (or 2nd-init recoveries in general) & all the work done by @Hashcode is GREATLY appreciated.
Click to expand...
Click to collapse
How can it overwrite system files while running?
MultiSystem allows you to install safe mod's or a ROM in full or OTA-like update. It's strongly recommended to install .ZIP files NOT to the current system, b/c some files can not be overwritten while running. So, you can use backup function to copy the current system & install to the new img or any of your other virtual systems. You'll have several options to activate a virtual img & reboot directly to stock system, any virtual img you've activated, quick reboot, Download/bootloader, recovery,... etc.
Click to expand...
Click to collapse
How would I benefit from it if I'm only running Stock ROM or would there be no point for me to install it?
If you run a ROM on stock system, you're vulnerable to root loss unless/untill a new rooting method for LP comes out. MultiSystem gives you the option to run safe-to-mod virtual ROMs + recovery replacement + extra features.
Click to expand...
Click to collapse
Is there a way to convert a normal ROM .ZIP into MultiSystem .IMG?
Create or copy any of your IMGs, activate it & reboot to the active IMG! Then, use FlashFire to flash the ZIP file. However, the updater-script should be safe/compatible. Some devs mount the phyical partition, which will redirect everything to it!!
For example:
Code:
mount(“ext4″, “EMMC”, “/dev/block/mmcblk0p23″, “/system”);
will mount the original system partition; while
Code:
run_program("/sbin/mount", "-t", "auto", "/system");
will mount the current system (stock or virtual). This is recommended/safe.
Click to expand...
Click to collapse
Would a KitKat ROM work with multisystem even though my stock is Lollipop?
Any ROM requires a compatible kernel & modem. So, running KK ROMs requires flashing KK firmware (namely, kernel & modem). This may work with MultiSystem on other devices, especially if the bootlpoader is unlocked. For example, I plan to add features for Note 4 DevED to allow different Android versions (including AOSP, manufacturer-based, & probably Linux systems) by utilizing kernel swapping or execution.
Click to expand...
Click to collapse
When MultiSystem comes out will it be open sourced?
Most probably, haven't decided yet!
Anyway, here's the repository on GitHub: https://github.com/hsbadr/MultiSystem
Click to expand...
Click to collapse
Click to expand...
Click to collapse

Video Tutorials
A quick preview of MultiSystem v1.0 tested on Lollipop for VZW Note 3. The video has been captured on a stable virtual ROM of JasmineROM v5.0.1. It's FULLY compatible with FlashFire on virtual/stock systems. More devices will get supported as well, after required testing.
Facebook: https://www.facebook.com/hsbadr/videos/vb.331488823689599/428178174020663

How to check if you are running a Stock/Virtual System?
There're many ways to check whether you're running a Stock or Virtual system. MultiSystem app should include this simple check at some point. That's important to avoint ruining the Stock system & keep it safe. To make it clear to NOOBZ & anyone who's requesting "another" proof even though I owe hime nothing. Very weird!
Anyway, BusyBox mountpoint applet can print the current block/device mounted to /system mountpoint by running the following command:
Code:
busybox mountpoint -n /system
The stock system is mounts the original system partition:
Code:
[B][COLOR="Red"]/dev/block/mmcblk0p23[/COLOR][/B]
while the virtual system mounts a loop device associated with a system IMG:
Code:
[B][COLOR="Blue"]/dev/block/loop0[/COLOR][/B]
Here're two videos for both stock & virtual systems...
UPDATE:
Now, you could run the following command to print the current system (stock or virtual) and the system device (physical partition or loop device):
Code:
MultiSystem status
Note: The block device number (mmcblk0p23) may vary per device & per variant !

How to repartition microSD card for MultiSystem?
You can use any tool/program for partitioning on Android, Linux, Mac, or Windows. For example, MiniTool Partition Wizard is a good partitioning tool for Windows. So, let's use it for this task. Simply, you need to follow this PDF tutorial (thanks to @carl1961). In sum:
Step 1: delete old partitions on SD card
Step 2: create FAT32 PRIMARY partition
Step 3: create EXT4 PRIMARY partition
Then, apply changes (note that the program UI may get changed in newer versions).
Notes:
This partitioning tutorial doesn't create PRIMARY partitions (it creates logical partitions). So, you need to change "Create As" from "Logical" to "Primary" when creatig a partition.
The sizes of the two partitions are arbitrary depending on number of ROMs you plan to install on the 2nd EXT4 partition.
The 1st partition (check size) is automatically detected as your external storage
In Terminal Emulator or ADB shell, check the existence of the two partitions by running the following command (in red):
Code:
[email protected]:/ # [COLOR="Red"]ls -l /dev/block/platform/msm_sdcc.3/[/COLOR]
drwxr-xr-x root root 2015-05-02 21:08 by-num
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1 -> /dev/block/mmcblk1
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p1 -> [COLOR="Blue"]/dev/block/mmcblk1p1[/COLOR]
lrwxrwxrwx root root 2015-05-02 21:08 mmcblk1p2 -> [COLOR="Blue"]/dev/block/mmcblk1p2[/COLOR]
/dev/block/mmcblk1p1 is mounted by Android as your external storage.
/dev/block/mmcblk1p2 is NOT mounted & will be your MultiSystem partition.
Click to expand...
Click to collapse

How to check microSD card partitions for MultiSystem?
You need to correctly repartition microSD card into two partitions:
exFAT or FAT32 for the 1st partition (your new external storage)
EXT4 for the 2nd partition (your MultiSystem partition)
Use the directions in this post!
You should check your 2nd SD partition in EXT4 format mounted to /MultiSystem:
check that the /MultiSystem directory exists after a reboot
check that the 2nd SD partition (/dev/block/mmcblk1p2) is mounted to /MultiSystem by running the following command in Terminal Emulator or ADB shell:
Code:
mount | grep /MultiSystem
The output should be:
Code:
/dev/block/mmcblk1p2 /MultiSystem ext4 rw,seclabel,relatime,data=ordered 0 0

How to check MultiSystem Installation?
The 1st thing to do after installing MultiSystem is to check the /MultiSystem directory & its contents (it shouldn't be empty!). Then, check usage by running the following commands in Terminal Emulator or ADB shell:
Code:
su
bash
MultiSystem
If it retuns "MultiSystem not found" or permission denied, try to use open MultiSystem app to Update Configurations & try again. If this does't fix it, try the following command:
Code:
/MultiSystem/bin/MultiSystem
This should work if you've MultiSystem binaries installed in (extracted to) /MultiSystem directory. If so, you can create a symlink in /system/xbin as follows:
Code:
mount -o remount,rw /system
ln -sv /MultiSystem/bin/MultiSystem /system/xbin/MultiSystem
Then, test it by running:
Code:
MultiSystem
The last thing before using it is to check the boot options: reboot & monitor the GREEN LED indicator for 3 seconds (change in the app) , which give you the following options:
Volume UP = Primary virtual ROM
Volume DOWN = Secondary virtual ROM
HOME KEY = Stock System
Pressing nothing will boot the active system: stock or primary virtual system. Sure, you should have installed one or more virtual ROMs.
Backup & restore or creating/installing a virtual ROM are easy as copy & paste: all img's will be at
Code:
/MultiSystem/img/system
To backup a virtual/stock system, you have many options:
Use create function to create from stock system
Use copy function to copy the IMG
Copy & paste with a new name
Use FlashFire (fully supported on virtual/stock ROMs)
...
If you've IMG mounting issues, run the following commands:
Code:
mount -o remount,rw /system
busybox ln -sv /proc/self/mounts /system/etc/mtab
If this doesn't help, try mounting from Terminal Emulator or ADB shell after selecting the IMG in MultiSystem app, by running the following command:
Code:
MultiSystem mount virtual

MultiSystem Video Tutorial
Thanks To: @Tomsgt , aka RootJunky
Don't forget to subscribe & like the video to show appreciation of his great effort & time spent in making the video :highfive::good: