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RE: [Xen-users] question to DRBD users/experts

To: "James Pifer" <jep@xxxxxxxxxxxxxxxx>, "Xen List" <xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] question to DRBD users/experts
From: "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
Date: Thu, 26 Aug 2010 09:52:20 +1000
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Thread-topic: [Xen-users] question to DRBD users/experts
> > I'm using DRBD. I was using LVM2 on a multiple-primary DRBD (eg one
> > DRBD volume cut into slices with LVM) and when it worked it was fine
> > it would split brain occasionally (on startup after a crash
> > not just spontaneously) and the CLVM daemon would hang on occasion
> > no good reason.
> >
> > Now I'm using DRBD on LVM on RAID0 and only multiple-primary where
> > necessary. Each DRBD is formed from an LV on each node. Extra work
> > create a new DRBD volume (create LV on both nodes then set up the
> > but much less likely to go wrong during normal use - it hasn't gone
> > wrong yet after months of use!
> >
> -Ok, so:
> -You create the same size LV on each xen host.
> -Setup a DRBD using that LV
> -Each VM would use that DRBD as it's storage?

Correct. It might be a bit of a pain to resize an LV but I haven't had
to try yet.

> In a split brain you then choose which way to recover the data for
> separate LV/DRBD set.

I'm only using a single primary model now so the problem hasn't come up.

> How difficult/complex is it for you to add a VM this way? I guess once
> you get the procedure down it's probably not that difficulty...

It's tricker than having LVM on top of DRBD but it's not so bad.

> Any experience with ocfs2 over drbd? In our testing it has actually
> quite stable, and at times even tough to force a split brain
> but you never know when it's going to happen!

No. And in fact you'd be dealing with the possibility of split brain on
DRBD and on OCFS2 so it becomes even trickier.

> > A better setup though would be a SAN consisting of iSCSI on DRBD in
> > single primary mode (using HA to handle failover if the primary
> > and all the hosts using iSCSI. I don't have enough hardware to make
> > work though unfortunately.
> Using a SAN would be our first choice, unfortunately the costs, even
> a low end SAN, make it not possible.

I've not yet done any performance tests to see if what I can build out
of low end equipment would be fast enough...


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