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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Wed, Jan 26, 2011 at 12:15:40PM -0800, Freddie Cash wrote:
> On Wed, Jan 26, 2011 at 12:05 PM, Christian Zoffoli
> <czoffoli@xxxxxxxxxxx> wrote:
> > Il 26/01/2011 18:11, Freddie Cash ha scritto:
> > [cut]
> >> If there's anything that we've missed, let me know.  :)
> >
> > the exposed setup is very intesting but:
> >
> > a) ZFS on freebsd is not as stable as on solaris
> 
> But it's plenty stable enough for our uses.  Been using it for over 2
> years now, since ZFSv6 hit FreeBSD 7.0.  Once we got over the initial
> tuning glitches and upgraded to ZFSv14, things have been rock solid,
> even when swapping drives.
> 
> > b) opensolaris is dead (oracle killed it)
> > c) we have no guarantee that in the future oracle will release updated code
> 
> For ZFS?  No, there are no guarantees.  But the Illumos, Nexenta, and
> FreeBSD devs won't be sitting still just waiting for Oracle to release
> something (look at the removal of the python dependency ZFS
> delegations in Illumos, for example).  This may lead to a split in the
> future (Oracle ZFS vs OpenZFS).  But that's the future.  ZFSv28 is
> available for FreeBSD right now, which supports all the features we're
> looking for in ZFS.
> 
> > d) NFS is slow ...NFS over RDMA is fast but freebsd has no open/official
> > infiniband stack
> 
> NFS doesn't have to be slow.
> 
> > e) consistent snapshots are very different from backuping only files.
> > For example if you backup a DB server copying files is not enought you
> > have to dump also what you have in memory at the same time (the key word
> > is "at the same time")
> 
> Yes, true.  But having a cronjob in the guest (or having the backups
> server execute the command remotely) that does a dump of the database
> before the backup snapshot is created is pretty darn close to atomic,
> and hasn't failed us yet in our restores.  It's not perfect, but so
> far, so good.
> 
> Compared to the hassle of getting iSCSI live-migration working, and
> all the hassles of getting a cluster-aware LVM or FS setup, I'll take
> a little drop in raw disk I/O.  :)  Ease of management trumps raw
> performance for us (we're only 5 people managing servers for an entire
> school district of ~2100 staff and 50 schools).
> 

You don't need CLVM for live migration on shared iSCSI+LVM..

See for example Citrix XenServer (or XCP): they use just the normal
LVM on top of shared iSCSI, and live migrations work perfectly OK.

The trick is that the XAPI toolstack takes care of the locking,
so there's no need for CLVM.

-- Pasi


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