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Re: [Xen-users] Shared storage and file-based VHDs

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Subject: Re: [Xen-users] Shared storage and file-based VHDs
From: Bart Coninckx <bart.coninckx@xxxxxxxxxx>
Date: Mon, 18 Oct 2010 10:19:12 +0200
Cc: Craig Miskell <craig.miskell@xxxxxxxxxx>
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On Monday 18 October 2010 05:27:03 Craig Miskell wrote:
> Hi,
>       This is related to the recent thread about best practices in using
> shared storage, but coming at it from a slightly different angle.
> I'm setting up a pre-production/test environment using XCP; with how we
> plan on operating this system, there's going to be some pretty rampant
> snapshots and cloning of some reasonably large VHDs.  As such, I want to
> use file-based VHDs rather than LV-based, in order to take advantage of
> thin-provisioning to minimise disk space.  I'm happy with the performance
> hit this causes.
> Further, I want to use shared storage so that I can have multiple hosts and
> can easily expand processing capacity as we spin up various instances, and
> do migrations.  However, I'm not using shared storage for auto failover or
> hot spare type functionality; migration will be manually managed as
> required.
> So, from what I've been reading, I think I need one of the following two
> options:
> 1) NFS.  Simple, understood technology.  Low overhead, and the XAPI
> toolstack takes care of "sharing" the VHDs.
> 2) iSCSI, GFS(2), cLVM.  Storage LUN(s) presented by iSCSI, turned into an
> LV using cLVM, formatted with GFS or GFS2, and this filesystem added as a
> "file" type SR.  More complicated than NFS, and I've read there were some
> problems with GFS in this sort of scenario, to do with mounting via the
> loopback device.  But that was back a few years, and may have been solved,
> either in GFS or in GFS2.
> Have I missed any other options?  Just pointers in the right direction
> (keywords) is enough if that's all you've got time for.
> Is there anything glaringly wrong with my briefly written understanding of
> the options?
> And does anyone have any comments on which is likely to be better?
> Thanks,

Ow yes, I use iSCSI with nothing on it, as I use shared block devices, not 
image files. It rules out the complexity of GFS, cLVM or OCFS2. You do need a 
clustering software to prevent guest booting from the same storage twice.

Good luck,


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