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

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] domU disk image over NFS
From: "Tomoki Taniguchi" <tomoki.taniguchi@xxxxxxxxx>
Date: Thu, 16 Aug 2007 00:48:36 +0900
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  thank your for your suggestion.
  I am not familiar with nbd, I was hoping you could help me out a little more.

  can you tell me what the difference is between enbd and gnbd?  which
is better?

  does xen support gnbd?

  from what i read about nbd it seems to export whole partitions, but
can it export folders like nfs or smb?

  is nbd able to export a loop back filesystem?


On 8/13/07, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > 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,
> Cheers,
> Mark
> --
> 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!

Tomoki Taniguchi
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