OTA update vs Image update

Question

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

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).}