On Mon, Aug 1, 2016 at 10:08 AM, Austin S. Hemmelgarn
[off-list ref] wrote:

MD and DM RAID handle this by starting kernel threads to do the scrub. They
then store the info about the scrub in the array itself, so you can query it
externally.  If you watch, neither of those commands runs longer than it
takes to start the operation, so there's nothing for systemd to kill.

pvmove continues to run and report progress so it can be killed off,
but it only polls for statistics, it's not actually recording them. So
even though it gets killed, subsequent pvmove command shows correct
statistics.

So that makes me wonder how btrfs device add and remove will behave,
if issued in a DE which is then logged out of. Those commands do not
return to prompt until they complete.

Replace was implemented the way scrub should have been.  It's done entirely
in the kernel, and the userspace tools just start, stop and check status.
We should just get rid of the whole scrub state file crap and have a way to
query the last scrub status directly from the FS. That would fix this
particular issue, and make scrub more consistent with everything else (and
solve the stale scrub status bug too).

OK, I'll update the bug report.

-- 
Chris Murphy