Re: [Xen-users] Cluster xen servers with san storage

["Tijl Van den Broeck" <subspawn@xxxxxxxxx>]

For the answers I start from the point of view of hearbeat 2
(www.linux-ha.org). I do not know about commercial cluster products
supporting Xen. Crossposting this to linux-ha mailing list as they can
correct me on possible mistakes :-)

On 3/28/07, Carl Caum <sleepkreep@xxxxxxxxx> wrote:
> Hello all.  I have two servers with Xen running.  I also have  a SAN
> that hold the virtual machines.  I need to know if a few things are
> possible.
>
> 1)  I need to know if failover is possible and how is the best way to
> accomplish this?  Is it possible to have one dom0 take snapshots of
> the other dom0 and restore the VMs from the other dom0 if it suddenly
> stops responding?
In theory yes, if you wrote your own necessary external plugins for
doing so. But I presume you don't want to go through those troubles as
there already is a native Xen OCF Resource Agent in the heartbeat
project. A good explanation & demonstration for it can be found in the
"SUSE Linux Enterprise Server 10 - Exploring the High-Availability
Storage Foundation" presentation from september 2006, more
specifically pages 137-142. It should also answer questions you might
have. However if you really want to reinvent the wheel, please stick
to the guidelines mentioned in the linux-ha wiki.

> 2)  Is it possible to have a VM do a live migrate to the other server
> automatically if the current server it's running on is being shutdown
> for maintenance.
Live-migration is not yet included in the Xen OCF RA, only "migrate"
is possible at the moment. Also do NOT trigger a live-migration
yourself using "xm", as hb2 will think the resource crashed and will
try to start it again which leads to ehm... data corruption to say at
least :-)

> 3)  Is it possible to cluster the two servers so they share CPU power
> and memory resources between the two instances of xen.  So really the
> VMs are arbitrarily run on both servers simultaneously thus
> increasing their speed.  If this is possible, what happens if one of
> the servers suddenly crashes?
By this you mean... running the same machine in parallel, whilst
sharing exclusive resources as such would be possible by an
Active-Active cluster? This would not really be a Xen issue, rather a
hb2 one with the use of a cluster filesystem, not everything can run
in A-A mode I think (not an expert on this, check the linux-ha wiki
for more info).
To accomplish this, additionally to setting up hb2 between your
dom0's, you'd also have to set it up between the domU's running inside
the dom0's.

There have been discussions on the linux-ha list about the Xen RA (and
someone who was writing an alternative one for domU's specifically I
think), I suggest you find 'm & read :-)


Tijl Van den Broeck

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