Re: Making snapshot of logical volumes handling HVM domU causes OOPS and instability

[Scott Garron <xen-devel@sce.pridelands.org>]

On 9/12/2010 5:41 AM, J. Roeleveld wrote:
> I also use LVMs extensively and do similar steps for backups.
> 1) umount in domU
> 2) block-detach
> 3) lvcreate snapshot
> 4) block-attach
> 5) mount in domU

      I think the biggest difference, here, is that you unmount and
detach the source volumes before creating the snapshot whereas I just
leave them active and mounted in the guest.  I don't know if that will
end up being the difference between stability and instability on my
system, but it's an observation and probably worth experimentation.

> I, however, have no need for HVM and only use PV guests.

      It turns out that it doesn't seem isolated to HVM guests on my
system any longer.  That was just coincidental during the first few
crashes that I observed.

> Are you certain the snapshots are large enough to hold all possible
> changes that might occur on the LV during the existence of the
> snapshot?

      Certainly.  The most recent one to cause a crash has existed
through the crash and for 3 days now, and is only using 2.65% of its COW
space.  They usually don't get a chance to go above even 0.3% before the
rsync on them is finished and they are unmounted and removed by the
backup script.

> Another thing I notice, which might be of help to people who
> understand this better then I do, in my backup-script, sometimes step
> "5" fails because the domU hasn't noticed the device is attached
> again when I try to mount it. The domU-commands are run using
> SSH-connections.

      That probably just has to do with variations in how long it takes
the guest kernel to poll or be notified of device changes, and how long
it takes for its udev to create the device files and whatnot.
Introducing some sanity checks or just a longer delay in your backup
script would likely get around that problem.  (I could be wrong, though)

-- 
Scott Garron