[Linux-cluster] XEN and Cluster Questions

[Lon Hohberger]

On Wed, 2007-10-17 at 11:41 +0200, Marc Grimme wrote:
> Hello,
> we are currently discussing XEN with clustering support. 
> There came some questions we are not sure what the answer is. Perhaps you can 
> help ;-) .
> 
> Background is: We are discussing a group of XEN Dom0 Hosts sharing all devices 
> and files via GFS. They themselves again host a couple of virtually 
> redhat-clustered DomU Hosts with or without gfs.
> 
> 1. Live Migration of cluster DomU nodes:
> When I live migrate a virtual DomU clusternode to another DOM0 XEN Host the 
> migration works ;-) , but the virtual clusternode is thrown out of the 
> cluster. Is this a "works as designed"? I think the problem are the 
> heartbeats not coming in proper time.
> Does that lead to the conclusion that one cannot live migrate cluster nodes?

Depends.  If you're using rgmanager to do migration, the migration is
actually not live.  In order to do live migration,
change /usr/share/cluster/vm.sh...

  - where it says 'xm migrate ...'
  - change it to 'xm migrate -l ...'

That should enable live migration.

> 2. Fencing:
> How about fencing of the virtual Dom-U Clusternodes. You are never sure on 
> which Dom-0 Node runs our Dom-U Clusternode. Is the fencing via fence_xvm[d] 
> supported on such an environment? That means how does a virtual DomU 
> clusternode X running on Dom0 Xen Host x know that if virtual DomU 
> clusternode Y running on Dom0 Xen Host y is running there when it is getting 
> the fence request to fence Host y where it is running?

Yes.  Fence_xvmd is designed (specifically) to handle the case where the
dom0 hosting a particular domU is not known.  Note that this only works
on RHEL5 with openais and such; fence_xvmd uses AIS checkpoints to store
virtual machine locations.

Notes:
* the parent dom0 cluster still needs fencing, too :)
* do not mix domU and dom0 in the same cluster,
* all domUs within a dom0 cluster must have different domain names,
* do *not* reuse /etc/xen/fence_xvm.key between multiple dom0 clusters

-- Lon