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Re: [Xen-users] guest OS High Availability with local storage

  • To: xen-users@xxxxxxxxxxxxx
  • From: John Sherwood <jrs@xxxxxx>
  • Date: Tue, 4 Sep 2012 22:01:33 -0700
  • Delivery-date: Wed, 05 Sep 2012 05:02:26 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

On Tue, Sep 4, 2012 at 9:44 PM, Fajar A. Nugraha <list@xxxxxxxxx> wrote:
On Wed, Sep 5, 2012 at 10:57 AM, Umarzuki Mochlis <umarzuki@xxxxxxxxx> wrote:
> hi,
> if I were to use Debian Squeeze as dom0 (3 servers) and install Xen
> hypervisor 4.0 on all of them, how would I achieve HA with only using
> local storage?

You can't

> has it ever been done before?

The easiest way would be to use shared storage and some sort of
cluster manager. So if it were me I'd use one node as SAN (e.g. with
openindiana + nappit, or whatever iscsi-capable storage appliance of
your choice) and the other two as compute nodes. The SAN becomes a
single point of failure though (unless you can afford two storage
nodes, but that's another story).

If you don't have SAN, then to have a "shared" common storage you can
use DRBD. AFAIK you can only have two nodes for primary-primary setup
though. See https://alteeve.ca/w/Red_Hat_Cluster_Service_3_Tutorial
for example (it uses KVM, but the concept should be similar).


You can, technically do this with only local storage and stuff like DRBD; boot your HA host off the network and store everything in RAM, bypassing local storage entirely. The drawback there is obvious, but it is a (VERY application dependent) option.

Also, with respect to doing it using Xen, Xen HA = Remus, and DRBD is going to be your best option for storage if you've ruled out an actual HA storage solution (e.g., a SAN). It's discussed in the original Remus whitepaper as well (http://nss.cs.ubc.ca/remus/papers/remus-nsdi08.pdf). That being said, while you can technically make DRBD work across three hosts (mirror A to B, then B to C), I'm not sure if you could make Remus do that, and on the subject of Remus, I wouldn't trust it in prod until 4.2 is released.

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