Re: ENOSPC errors during balance

[Marc Joliet <marcec@gmx.de>]

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Am Tue, 22 Jul 2014 00:30:57 +0200
schrieb Marc Joliet <marcec@gmx.de>:

> Am Mon, 21 Jul 2014 15:22:16 +0200
> schrieb Marc Joliet <marcec@gmx.de>:
> 
> > Am Sun, 20 Jul 2014 21:44:40 +0200
> > schrieb Marc Joliet <marcec@gmx.de>:
> > 
> > [...]
> > > What I did:
> > > 
> > > - delete the single largest file on the file system, a 12 GB VM image, along
> > >   with all subvolumes that contained it
> > > - rsync it over again
> > [...]
> > 
> > I want to point out at this point, though, that doing those two steps freed a
> > disproportionate amount of space.  The image file is only 12 GB, and it hadn't
> > changed in any of the snapshots (I haven't used this VM since June), so that
> > "subvolume delete -c <snapshots>" returned after a few seconds. Yet deleting it
> > seems to have freed up twice as much. You can see this from the "filesystem df"
> > output: before, "used" was at 229.04 GiB, and after deleting it and copying it
> > back (and after a day's worth of backups) went down to 218 GiB.
> > 
> > Does anyone have any idea how this happened?
> > 
> > Actually, now I remember something that is probably related: when I first
> > moved to my current backup scheme last week, I first copied the data from the
> > last rsnapshot based backup with "cp --reflink" to the new backup location, but
> > forgot to use "-a".  I interrupted it and ran "cp -a -u --reflink", but it had
> > already copied a lot, and I was too impatient to start over; after all, the
> > data hadn't changed.  Then, when rsync (with --inplace) ran for the first time,
> > all of these files with wrong permissions and different time stamps were copied
> > over, but for some reason, the space used increased *greatly*; *much* more than
> > I would expect from changed metadata.
> > 
> > The total size of the file system data should be around 142 GB (+ snapshots),
> > but, well, it's more than 1.5 times as much.
> > 
> > Perhaps cp --reflink treats hard links differently than expected?  I would have
> > expected the data pointed to by the hard link to have been referenced, but
> > maybe something else happened?
> 
> Hah, OK, apparently when my daily backup removed the oldest daily snapshot, it
> freed up whatever was taking up so much space, so as of now the file system
> uses only 169.14 GiB (from 218).  Weird.

And now that the background deletion of the old snapshots is done, the file
system ended up at:

# btrfs filesystem df /run/media/marcec/MARCEC_BACKUP    
Data, single: total=219.00GiB, used=140.13GiB
System, DUP: total=32.00MiB, used=36.00KiB
Metadata, DUP: total=4.50GiB, used=2.40GiB
unknown, single: total=512.00MiB, used=0.00

I don't know how reliable du is for this, but I used it to estimate how much
used data I should expect, and I get 138 GiB.  That means that the snapshots
yield about 2 GiB "overhead", which is very reasonable, I think.  Obviously
I'll be starting a full balance now.

I still think this whole... thing is very odd, hopefully somebody can shed
some light on it for me (maybe it's obvious, but I don't see it).

-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup

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