On 02/09/2018 05:45 PM, Ellis H. Wilson III wrote:
> I am trying to better understand how the cleaner kthread (btrfs-cleaner)
> impacts foreground performance, specifically during snapshot deletion.
> My experience so far has been that it can be dramatically disruptive to
> foreground I/O.
> Looking through the wiki at kernel.org I have not yet stumbled onto any
> analysis that would shed light on this specific problem.  I have found
> numerous complaints about btrfs-cleaner online, especially relating to
> quotas being enabled.  This has proven thus far less than helpful, as
> the response tends to be "use less snapshots," or "disable quotas," both
> of which strike me as intellectually unsatisfying answers

Well, sometimes those answers help. :) "Oh, yes, I disabled qgroups, I
didn't even realize I had those, and now the problem is gone."

>, especially
> the former in a filesystem where snapshots are supposed to be
> "first-class citizens."

Throwing complaints around is also not helpful.

> The 2007 and 2013 Rodeh papers don't do the thorough practical snapshot
> performance analysis I would expect to see given the assertions in the
> latter that "BTRFS...supports efficient snapshots..."  The former is
> sufficiently pre-BTRFS that while it does performance analysis of btree
> clones, it's unclear (to me at least) if the results can be
> forward-propagated in some way to real-world performance expectations
> for BTRFS snapshot creation/deletion/modification.

I don't really think they can.

> Has this analysis been performed somewhere else and I'm just missing it?
>  Also, I'll be glad to comment on my specific setup, kernel version,
> etc, and discuss pragmatic work-arounds, but I'd like to better
> understand the high-level performance implications first.

The "performance implications" are highly dependent on your specific
setup, kernel version, etc, so it really makes sense to share:

* kernel version
* mount options (from /proc/mounts|grep btrfs)
* is it ssd? hdd? iscsi lun?
* how big is the FS
* how many subvolumes/snapshots? (how many snapshots per subvolume)

And what's essential to look at is what your computer is doing while you
are throwing a list of subvolumes into the cleaner.

* is it using 100% cpu?
* is it showing 100% disk read I/O utilization?
* is it showing 100% disk write I/O utilization? (is it writing lots and
lots of data to disk?)

Since you could be looking at any combination of answers on all those
things, there's not much specific to tell.

Hans van Kranenburg
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