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Re: [Linux-cluster] Xen DomU as Cluster service?



On Thu, 2008-02-14 at 09:15 +0100, Gunther Schlegel wrote:
> Hello,
> 
> In short: Is it a good idea to run Xen DomUs as a cluster service or is 
> there an easier way to do it?

While it is not particularly hard, we're working on an easier way,
too ;)

https://bugzilla.redhat.com/show_bug.cgi?id=432640


> -* If a Dom0 goes is shut down the DomUs need to be live migrated to 
> another Dom0 host during Dom0 shutdown.

This isn't done automatically from the cluster software at this point.
You should migrate VMs before shutting the machine down, or they will be
offline for a few seconds.  There's an open bugzilla (feature request)
to make this happen automatically, but it will always be safe to do the
migrations manually.


> -* If a Dom0 dies unexpectedly the DomUs need to be restarted on 
> antother hot.

That can be done; just set recovery to 'relocate' instead of
'restart' (the default) as part of the service configuration.


> Problems / open Questions:
> - xen supports life migration on system shutdown, but only to a specific 
>   host. As I will have a couple of hosts it is more desireable to 
> distribute the DomUs among the other Dom0 hosts instead of migrating 
> them all to the same Dom0 host. Has anyone done that before? Is it a 
> good option? Are there other options?

Since you know you're shutting a particular host down and are doing the
migration as part of the shutdown process, you can decide where they go.


> - I suppose I need the full cluster stuff anyways to deal with a crashed 
> Dom0, this implies to define a cluster service for each DomU?

Not necessarily, unless you want to manage migrations/where things start
yourself.

Generally speaking, you *do* need a cluster running for the domUs to be
clustered (unless you running all domUs on one host...) - even if you
are not using rgmanager to manage the domUs.

This is because fence_xvmd (libvirt-based fencing host) stores domU
states in distributed AIS checkpoints, which are accessed when a fencing
request comes in.

-- Lon


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