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xen-users
Re: [Xen-users] Advice on redundant SAN/NAS storage for Xen
On Sat, May 30, 2009 at 3:30 AM, Chris 'Xenon' Hanson
<xenon@xxxxxxxxxxxxxx> wrote:
> I'm planning to expand my Xen servers at my datacenter into a cluster with
> high
> availability and reliability. As part of this, I want to move all DomU
> storage to a common
> SAN or NAS infrastructure and make all the Dom0s basically identical. In this
> way, I can
> move DomU's around between Dom0s as needed for performance or reliability
> reasons. If a
> Dom0 server fails, I can just bring up its DomUs on different servers with no
> loss.
Simple goal, not-so-simple implementation.
> The best design I can think of is this:
>
> Two machines running Linux configured as SANs, using something like ATA over
> Ethernet
> (AoE) to link them to a pair of GigE switches that then link to every Dom)
> box. The pair
> of SAN boxes each export a block of raw storage that the Dom0 machine then
> RAIDs together
> as RAID1 and provides to Xen and the DomU as a block device. The Dom0 gets
> network-portable storage, with RAID reliability and redundancy.
>
> The other way might be to have the Dom0 and Xen pass through both block
> devices to the
> DomU and let the DomU RAID them together. I'm not sure if either is better.
> Maybe RAID on
> the DomU would allow the DomU to be migrated easier?
RAID might be the weakest link here. Think what will happen if :
- one of the SAN box gets disconnected -> RAID will (hopefully) cope
with it well and use the live SAN
- some time later, the dead SAN is available again -> RAID won't
automatically re-add it
- the other SAN dies.
These are big IFs, but you get the idea.
>
> Is there a better and less messy way to provide redundant SAN-type storage
> to Xen DomUs?
> The main criteria are:
>
> Immune to failure of a single switch or SAN box.
> Allow DomUs to be moved seamlessly to other Dom0s without messy
> reconfiguration.
Immune to a SAN box failure is hard.
The common way to do it in enterprise-level storage is to have high
availability in the SAN box. It does raid and have multiple
controllers in a cluster/HA setup so that it'd be "immune" enough to
disk or controller failure. I don't think there's a viable way to
achieve that with your planned setup. Feel free to correct me if I
wrong.
--
Fajar
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