OTA update is defined as update to system's partition, but I read that OTA update is NOT a mere update from the last version.

So, what does it mean :

Does it contain all files in system.img (and other) partition ?

If so, isn't it quite the same as system.img (image of the system parition) ?

Regards, Ranchu

1 Answer 1


Does it contain all files in system.img partition ?..
[Isn't] it quite the same as system.img (image of the system parition) ?

  • System.img is a file whose contents are written into the partition named system (/system).
  • An OTA update is not necessarily restricted to updating a system partition alone. It concerns with anything of the firmware including boot, cache, bootloader, recovery, etc.
  • Not every time a full blown system.img (DAT file actually) is shipped to the end users just for every OTA update. Depending upon the type of update1, there may be a full image of system partition to be copied or just only the specific files (packed into a single file) that are to be copied. The latter type of update, called the incremental update, is the norm since Android 5.0.

In a nutshell, if you're thinking that every OTA update actually downloads a full system.img or equivalent file and writes it into system partition, then that is simply not the case.

1 An OTA update can just be a mere update (like bug fixes, vulnerability fixes) or altogether a platform upgrade (like from Android Marshmallow to Nougat).

  • Thanks that make thing more clear. So, when using OAT update is there an assumption that some previous OTA already been applied ?
    – ransh
    Commented Dec 3, 2016 at 11:52
  • The assumption is that the current setup of the end user is using a particular Android build (this is often the last build or the downloaded build). Before the OTA update is written this check is performed. If the build number doesn't match, the update fails. I've written about this at length here.
    – Firelord
    Commented Dec 3, 2016 at 12:04

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