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

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Subject: [Xen-users] Shared storage and file-based VHDs
From: Craig Miskell <craig.miskell@xxxxxxxxxx>
Date: Mon, 18 Oct 2010 16:27:03 +1300
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        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 

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

And does anyone have any comments on which is likely to be better?


- --
Craig Miskell
Senior Systems Administrator
Opus International Consultants
Phone: +64 4 471 7209
"Opportunity is missed by most people because it is dressed in overalls and
looks like work."
        -- Thomas A. Edison

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