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I am trying to backup my system but i have two partitions, system_image and system.. from inspection i can see that system_image is bigger and the format is ext4, While system is smaller and the format is emmc ..

I may have the formats (emmc,ext4) in the wrong order.

What do i back up, system or system_image ?

I also have system modifications that I'd like to keep in the backup.

Device : Android 5.1

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    "Android 5.1" is no device but OS and version. To answer the question it might help to know manufacturer and model of the device.
    – Izzy
    Apr 12 '18 at 8:41
  • It makes no difference what device I'm using, only the Android version is required to answer this. Apr 12 '18 at 13:05
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    If you say so. I have multiple devices, some of them were running Android 5.1. None of them had a partition named system_image. So if that should be specific to Android 5.1 and not to the device, something must be wrong with all of my devices. But it's your question, so I leave it at that. Good waiting then.
    – Izzy
    Apr 12 '18 at 13:20
  • What method did you use to view the partitions ? Apr 12 '18 at 13:23
  • See my answer on How to list all major partitions with their labels?. Not one method, but all that are available (to my knowledge).
    – Izzy
    Apr 12 '18 at 13:33
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this concerns TWRP only and is just a different method for backup & restore

have a look in /etc/recovery.fstab
you will see the same partition is listed twice

/system        ext4  /dev/block/bootdevice/by-name/system  flags=slotselect
/system_image  emmc  /dev/block/bootdevice/by-name/system  flags=slotselect

first entry is listed as ext4 file system type which tell TWRP the partition is mountable. in case of backup / restore only the content (files) are archived into system.ext4.win (tarball archive)

second entry is listed as emmc which makes TWRP think it is raw disk image. in case of backup / restore the whole partition (including empty space) is cloned bitwise into full partition image system.emmc.win (dd image)

usually /system is sufficient for backup purposes.
/system_image is useful for restoring foreign ROM or backup of corrupt data

Note: for system-as-root devices the mount point is changed into /system_root while /system is just a symlink (or bind mount) of /system_root/system (which is not a representative of whole partition)

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