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

To: "Javier Guerra Giraldez" <javier@xxxxxxxxxxx>, "Christian Zoffoli" <czoffoli@xxxxxxxxxxx>
Subject: RE: [Xen-users] iscsi vs nfs for xen VMs
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Sat, 29 Jan 2011 08:31:36 +1100
Cc: yue <ooolinux@xxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-users] iscsi vs nfs for xen VMs
> 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.

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