Re: Unable to allocate for space usage in particular btrfs volume

[Calvin Walton <calvin.walton@kepstin.ca>]

On Thu, 2015-11-05 at 10:44 +0000, OmegaPhil wrote:
> On 05/11/15 04:18, Duncan wrote:
> > OmegaPhil posted on Wed, 04 Nov 2015 21:53:09 +0000 as excerpted:
> > VM image files (and large database files, for the same reason) are
> > a bit 
> > of a problem on btrfs, and indeed, any COW-based filesystem, since
> > the 
> > random rewrite pattern matching that use-case is pretty much the
> > absolute 
> > worst-case match for a COW-based filesystem there is.
> > Since you're not doing snapshotting (which conflicts with this
> > option, 
> > with an imperfect workaround), setting nocow on those files may
> > well 
> > eliminate the problem, but be aware if you aren't already that (1)
> > nocow 
> > does turn off checksumming as well, in ordered to avoid a race that
> > could 
> > easily lead to data corruption, and (2) you can't just activate

> So a couple of gig still unaccountable but irrelevant. Thanks,
> problem
> solved! Although hopefully checksumming will be allowed on nocow
> files
> in the future as thats currently 17% of all data unprotected and will
> get worse...

There's actually an interesting workaround to this: Although the VM
disk images aren't checksummed on the host filesystem, you can use
btrfs *inside* the VMs and enable checksumming there. The downside is
that you can only verify the VM data by booting the VM and running a
scrub from inside.

This of course doesn't help if your VMs are Windows or legacy versions
of Linux without btrfs support. On BSD you could try ZFS.

-- 
Calvin Walton <calvin.walton@kepstin.ca>