On 2016-08-01 12:19, Chris Murphy wrote:

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

quoted

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.

Because all that the pvmove command is doing is polling for statistics. 
It actually works kind of like a scrub, all the actual work is done in 
the kernel, the userspace component just handles reporting.  The 
difference is that the move operation is accounted and mutexed in the 
kernel itself, instead of userspace like scrub does.  This model is 
actually essentially what I think scrub (and balance for that matter) 
should look like, and if implemented right, we could actually store 
scrub results in the FS itself (that is, in the metadata, not in special 
files or anything like that).

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.

They work via balance, so they should behave the same as a balance 
command, which means it will likely run part way then get cancelled 
because of the SIGTERM to the userspace component (assuming of course 
that it is still running when you log out).

quoted

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.