> -----Original Message-----
> From: linux-btrfs-ow...@vger.kernel.org [mailto:linux-btrfs-
> ow...@vger.kernel.org] On Behalf Of Graham Cobb
> Sent: Thursday, 19 May 2016 8:11 PM
> To: linux-btrfs@vger.kernel.org
> Subject: Re: [Not TLS] Re: Reducing impact of periodic btrfs balance
>
> On 19/05/16 05:09, Duncan wrote:
> > So to Graham, are these 1.5K snapshots all of the same subvolume, or
> > split into snapshots of several subvolumes? If it's all of the same
> > subvolume or of only 2-3 subvolumes, you still have some work to do in
> > terms of getting down to recommended snapshot levels. Also, if you
> > have quotas on and don't specifically need them, try turning them off
> > and see if that alone makes it workable.
>
> I have just under 20 subvolumes but the snapshots are only taken if
> something has changed (actually I use btrbk: I am not sure if it takes the
> snapshot and then removes it if nothing changed or whether it knows not to
> even take it). The most frequently changing subvolumes have just under 400
> snapshots each. I have played with snapshot retention and think it unlikely I
> would want to reduce it further.
>
> I have quotas turned off. At least, I am not using quotas -- how can I double
> check it is really turned off?
>
> I know that very large numbers of snapshots are not recommended, and I
> expected the balance to be slow. I was quite prepared for it to take many
> days. My full backups take several days and even incrementals take several
> hours. What I did not expect, and think is a MUCH more serious problem, is
> that the balance prevented use of the disk, holding up all writes to the disk
> for (quite literally) hours each. I have not seen that effect mentioned
> anywhere!
>
> That means that for a large, busy data disk, it is impossible to do a balance
> unless the server is taken down to single-user mode for the time the balance
> takes (presumably still days). I assume this would also apply to doing a RAID
> rebuild (I am not using multiple disks at the moment).
>
> At the moment I am still using my previous backup strategy, alongside the
> snapshots (that is: rsync-based rsnapshots to another disk daily and with
> fairly long retentions, and separate daily full/incremental backups using dar
> to a nas in another building). I was hoping the btrfs snapshots might replace
> the daily rsync snapshots but it doesn't look like that will work out.
I do a similar thing - on my main fs I have only minimal snapshots - like less
than 10. I rsync (with checksumming off and diff copy on) the fs to the backup
fs which is where all the snapshots live. That fs only gets the occasional 20%
balance when it runs out of space, and weekly scrubs. Performance doesn't seem
to suffer that way.
Paul.
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