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Re: [Xen-users] Stable VBD Types

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Stable VBD Types
From: Andy Smith <andy@xxxxxxxxxxxxxx>
Date: Mon, 20 Jun 2005 17:32:21 +0000
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On Mon, Jun 20, 2005 at 05:23:26PM +0100, [GDPR REDACTED] wrote:
> For a stable deployable, yet flexible solution, what VBD types do people
> here reommend.
> I know from the Documentation (and common-sense) that file-backed vbd's
> will die for I/O intensive activities, so I'm leaning towards LVM as a
> solution.

I use LVM for that reason and just because I'm familiar with LVM
alreayd and see no need to be adding file-backed VBDs into the mix.

> One thing that has caught my eye is the snapshot ability for a logical
> volume, but after a little more research and dipping into the archives
> I've heard nightmare stories with using snapshots with Xen. Is this
> still the case.

I'm currently using snapshotting for backup purposes, snapshotting
around 20 LVs every 20 minutes and mounting them in dom0.

I haven't had any problems with this except that with 256M RAM my
dom0 ran out of kernel memory and an lvcreate operation deadlocked.
I reported this to the LVM list and was told it was because I had
too little RAM.  After upgrading dom0 to 512M I have had no such
problems, although I still am not sure exactly how much RAM each
snapshot will need.

Today I saw a question on the linux-lvm list asking if snapshots
were considered stable in LVM2 now.  Maybe following that thread
would be more helpful than my anecdotes.

> Am I just better off simply installing domains as I need them? Thanks.

How do you mean?  If you start domains that you don't need then the
RAM is tied up in those domains.  If you wanted some other domain to
increase its RAM then it may be a laborious process taking a bit of
RAM from multiple other domains first.  Also the idle processes in
all those domains would take a small amount of CPU away.  Those are
about the only problems I can see with running domains you don't
immediately need.

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