Re: [Xen-users] Advice on redundant SAN/NAS storage for Xen

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

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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