Re: [PATCH v10 4/5] btrfs: test verity orphans with dmlogwrites

[Sweet Tea Dorminy <sweettea-kernel@dorminy.me>]

>> 
>> I'm not understanding the snapshot part. It seems like most tests 
>> using
>> log-writes do `_log_writes_replay_log_range $cur $SCRATCH_DEV >>
>> $seqres.full` to start each iteration; and then it seems like this 
>> test can
>> check the item counts before and after a 
>> _scratch_mount/_scratch_umount
>> cycle  and get the same results. (And, if that worked, the test 
>> wouldn't
>> need its own _cleanup() and its own lv management, I think?) But I'm
>> probably missing something.
> 
> IIRC, the purpose of the snapshots is that the mount/unmount cycle is
> destructive in the middle of the operation. If the orphan is present,
> we'll blow up all the verity items, so if we did it on the device we
> were replaying onto, it would leave it in a messed up state as we kept
> replaying. So we snapshot at each entry and mount/unmount that to check
> the invariants.

I think what you're saying is that we can't use the device itself 
instead of the snapshot, because mount/unmount change the underlying 
device, and this definitely makes sense.

Looking at other dmlogwrites users, though, generic/482 looks like it 
does something similar, and I don't understand what the difference 
between the replay+snapshot+mount cycles here and the replay+mount 
cycles there. I probably just don't understand what the difference 
between the two tests' scenarios is, though.