[lxc-users] ?==?utf-8?q? LXD - upgrade from 2.08 to 2.12 enables BTRFS quota

[Tomasz Chmielewski]

FYI btrfs quotas are not yet "stable" even in the newest kernel (4.11.x as we speak).

Reference: https://btrfs.wiki.kernel.org/index.php/Status

There are also btrfs quota problems mentioned on btrfs mailing lists quite often (severe performance problems, inaccurate or even negative counts, especially when snapshots are in use; kernel crashes/stability issues).


So making LXD enable it automatically might be premature - btrfs quotas might work in two or five years from now, but they are not there yet. If LXD wants to support it - fine; but IMO don't enable it by default, add a big fat warning for the user about the potential issues and document a way to enable it manually.


Tomasz Chmielewski
https://lxadm.com


On Monday, June 05, 2017 02:11 JST, Stéphane Graber <stgraber at ubuntu.com> wrote: 
 
> On Sun, Jun 04, 2017 at 08:36:28AM -0400, Ron Kelley wrote:
> > Greetings all.
> > 
> > I recently upgraded a number of LXD 2.08 servers to 2.12 and noticed the btrfs “quota” option was enabled where it was disabled before.  Enabling quotas on a filesystem with lots of snapshots can cause huge performance issues (as indicated by our 20min outage this morning when I tried to clear out some old snapshots).
> > 
> > Can one of the developers confirm if the upgrade enables quota?  If so, I will file a bug to ensure the user gets notified/alerted quotas will be enabled (so it can be disabled if necessary).
> > 
> > -Ron
> 
> LXD enables btrfs quota on any newly created btrfs pool, which includes
> all your existing ones when upgrading to LXD 2.9 or higher (as the
> upgrade step effectively creates a new pool).
> 
> That's done so that the size limit feature of LXD can work on btrfs.
> 
> 
> Given the potential performance impact and the fact that btrfs quotas
> aren't that great to begin with, I'd be fine with adding a pool
> configuration option that lets you turn them off. And tweaking our
> upgrade code to set that property based on the existing state.
> 
> 
> Filing a github issue would be good. Please include what I wrote above
> and I suspect Christian will take a look into this shortly.
> 
> -- 
> Stéphane Graber
> Ubuntu developer
> http://www.ubuntu.com