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Re: [Xen-users] iscsi vs nfs for xen VMs

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] iscsi vs nfs for xen VMs
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Sat, 29 Jan 2011 17:00:40 +0200
Cc: xen-users@xxxxxxxxxxxxxxxxxxx, yue <ooolinux@xxxxxxx>, Christian Zoffoli <czoffoli@xxxxxxxxxxx>, Javier Guerra Giraldez <javier@xxxxxxxxxxx>
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On Sat, Jan 29, 2011 at 08:31:36AM +1100, James Harper wrote:
> > 
> > 2011/1/28 Christian Zoffoli <czoffoli@xxxxxxxxxxx>:
> > > Il 28/01/2011 08:08, yue ha scritto:
> > >> what is the performance of clvm+ocfs2?
> > >> stability,?
> > >
> > > it's very reliable but not as fast as using clvm directly.
> > 
> > to expand a little:
> > 
> > ocfs2:
> > it's a cluster filesystem, it has the overheads of being a filesystem
> > (as opposed to 'naked' block devices), and of the clustering
> > requirements: in effect, having to check shared locks at critical
> > instants.
> Microsoft achieve high performance with their cluster filesystem. In fact the 
> docs clearly state it's only reliable for Hyper-V virtual disks, any other 
> use could cause problems, so I assume they get around the metadata locking 
> problem by isolating each disk file so there are no (or minimal) shared 
> resources.
> > 
> > clvm:
> > it's the clustering version of LVM.  since the whole LVM metadata is
> > quite small, it's shared entirely, so all accesses are exactly the
> > same on CLVM as on LVM.
> > 
> > the only impact is when modifying the LVM metadata
> > (creating/modifying/deleting/migrating/etc volumes), since _all_
> > access is suspended until every node has the a local copy of the new
> > LVM metadata.
> > 
> > Of course, a pause of a few tens or hundreds of milliseconds for an
> > operation done less than once a day (less than once a month in many
> > cases) is totally imperceptible.
> > 
> The dealbreaker for me with clvm was that snapshots aren't supported. I 
> assume this hasn't changed and even if it has, every write to a snapshotted 
> volume potentially involves a metadata lock so the performace drops right 
> down unless you can optimise for that 'original + snapshot only accessed on 
> the same node' case, which may be a limitation I could tolerate.

You don't really need CLVM for live migrations. Normal LVM works 
well on a shared iSCSI LUN. Assuming you know what you're doing..

It also allows you to use snapshots. See the other mail
I sent on this thread..

-- Pasi

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