[ovirt-users] Deleting large snapshots blocks the whole cluster

[Markus Stockhausen]

> Von: Stefan Wendler [stefan.wendler at tngtech.com]
> Gesendet: Montag, 27. Oktober 2014 11:39
> An: Markus Stockhausen
> Cc: users at ovirt.org
> Betreff: Re: [ovirt-users] Deleting large snapshots blocks the whole cluster
> 
> Hi,
> 
> do you mean during snapshot deletion or in general?
> 
> In general we do not have any swap usage at all. On none of the 4 hosts.
> 
> We had some swap I/O on the host that is SPM most of the time. But not
> during that period where we tried to delete the snapshot.
>

I thought of the bug described in RH bugzilla 1138690 "[SCALE] snapshot deletion -> 
heavy swapping on SPM". In case you have no access to it (as it is RHEV tagged): 

qemu-img reads through pagecache during snapshot deletion. A new flag has been 
intrduced in August this year that allows to avoid page cache. This flag must be
provided by OVirt/RHEV. Target release is 3.6. 

Until then the only bugfix is to drop pagecache manually on regular intervals on the 
hypversior and especially the SPM node.

Markus

> 
> Cheers,
> Stefan
> 
> On 10/27/14 11:08, Markus Stockhausen wrote:
> > Do you see swapping on the SPM? If yes a regular echo 3 > drop_caches could help.
> >
> > Markus
> >
> > Am 27.10.2014 10:57 schrieb Stefan Wendler <stefan.wendler at tngtech.com>:
> > Hi,
> >
> > we have some really large snapshots left from a migration. Since our
> > store is almost full now we have to delete them now.
> >
> > Some snapshots are around 1TB already.
> >
> > The last time we tried to delete a ~500GB snapshot the delete task
> blocked the whole diskstore's IO and the whole cluster and all hosts
> became unavailable.
>
> Is there a way to delete large snapshots in a "humane" way that will not
> block everything?
>
> Cheers,
> Stefan
>
>

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