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Re: [Xen-users] domU disk image over NFS

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Subject: Re: [Xen-users] domU disk image over NFS
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Mon, 13 Aug 2007 00:48:54 +0100
Cc: Tomoki Taniguchi <tomoki.taniguchi@xxxxxxxxx>
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> How safe is running a domU sda1 off of an image file located on an NFS?

If you can use it as a tap:aio it should work OK.  Don't use it as a file: 
disk (unless it's an HVM guest, in which case it's probably OK - I apologise 
but the disk handling is a bit weird at the moment!).

The reason for this advice is that file: VBDs for paravirt guests are 
implemented using losetup to bind the file to the loop device.  Doing this on 
an NFS mounted filesystem is known to cause nasty memory usage problems.

Hopefully even if this doesn't make sense to you right now, you'll see what I 
mean when you've got a bit further in setting stuff up ;-)  If not, folks on 
here should be able to fill you in.

> I am thinking about setting up a WEB server for hosting our internal
> web apps on
> a domU.  I am thinking about creating the root disk image on the NFS
> folder so that when i do i live migration the disk image is available
> on the new server.
> is this safe?

Should be fairly safe for HVM guests, or for paravirt guests using tap:aio...

I'm not sure whether the semantics of NFS will give you guaranteed data 
persistence (e.g. after a sync in the guest will the writes really have hit 
the disk at the server).

There used to be some race conditions with respect to storage when live 
migrating a guest, but they generally didn't seem to break things for 
anybody, and I think they may be fixed now anyhow.

Sorry for being a bit vague, I'm not so familiar with the specifics in this 
area at the moment.

> how is the performance?
> is there a better way to do this without investing in SAN?

Doing this over NFS is probably not optimal.  You could look at using ENDB 
(the Network Block Device).  The Xen config file format allows for remote NBD 
disks to be specified directly in the config file so that live migrations 
will automatically work properly, etc.

You need a separate machine (or virtual machine) to run as the NBD server, 
because it's not safe to mount an NBD server on the same machine it's running 
on.  Maybe you could run the NBD server on the storage server for your 
network, if it has the capacity to cope with the virtual machine disk 
traffic?  It's a bit less powerful than iSCSI but simpler to set up and it 
gives you similar "SAN over ethernet" functionality.

Hope that helps,


Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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