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that's how some phone OSes update the system (by having 2 read only fs)

that's how Chrome updates itself, but without the symlink part

No snapshotting at all? Thinking about it.. The filesystem does not support it I suppose.

Android does use snapshots: https://source.android.com/docs/core/ota/virtual_ab

Oh cool. I was a bit confused about not using snapshots and relying on symlinks but it couldn't be so simple. I guess it's just a simple userspace cow mount. https://source.android.com/docs/core/ota/virtual_ab#compress...

not surprised about the chrome part, but pretty shocked at the phone OS part. I know APFS migration was done in this way, but wouldn't storage considerations for this be massive?

what would be more massive would be phones not booting up because of a botched update. this way you can just switch back to the old partition

Not really, because only the OS core is swapped in this way. Apps and data live in their own partitions/subvolumes, which are mutable and shared between OS versions.

The OS core is deployed as a single unit and is a few GB in size, pretty small when internal storage is into the hundreds of GB.

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