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I recently play with LineageOS flashing (or actually adb-sideloading) tutorials, but I guess my question is more generic.

In my device there are A and B partitions for everything except the userdata partition (and maybe something else). I understand the purpose is to provide extra safety against bricking. Should anything go wrong during a flashing operation, there's the other slot.

Now, every tool and manual procedure I have seen flashes both A and B. Doesn't it render the whole concept useless? What's the gain from A and B slots if we keep on putting the same to both?

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    with "every tool and manual procedure" you mean unofficial usage only. it is used for OTA in right way
    – alecxs
    Commented Apr 28, 2020 at 22:00
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    Both slots aren't flashed at the same time. Inactive slot is flashed during an OTA update while the OS is running on active slot, without distribing user's workflow. If flashing goes well slots are switched on next reboot. If for some reason flashing or booting fails after update, bootloader falls back to previously active slot. And update is retried. When there is no update taking place, the inactive slot serves no purpose. You can keep a copy of active slot on it or erase the whole contents. It won't make any difference. Custom ROMs may or may not follow standard practices as alecxs said. Commented Apr 29, 2020 at 0:48
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    For more details on what @IrfanLatif said, see. It boils down to custom ROM implementation
    – beeshyams
    Commented Apr 29, 2020 at 7:33

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every tool and manual procedure I have seen flashes both A and B.

This is not correct-

Update engine does not flash both A and B, 2 slots exists for rollback purpose if updated slot does not boot up.

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