[virt-tools-list] question about clustering

[Daniel P. Berrange]

On Thu, Feb 17, 2011 at 09:10:01AM -0500, Kenneth Armstrong wrote:
> Ok, I'm trying to get my head wrapped around this:
> 
> 1.  I know that KVM apparently doesn't have the ability to share it's
> xml configuration files for the vm's across multiple nodes.  However,
> for a live migration to work, the xml file must exist on each node in
> order to migrate between them.  According to
> http://www.mail-archive.com/libvir-list@redhat.com/msg16921.html ,
> that is not a good idea.

This isn't entirely accurate. There are potentially 2 XML files
for any guest

 - A persistent guest has a XML file on disk in /etc/libvirt/qemu.
   If it is running it also has a live XML config in
   /var/lib/libvirt/qemu reflecting current state

 - A transient guest only has the live XML config in
   /var/lib/libvirt/qemu reflecting current state


During migration, libvirt automatically transfers the *live*
XML config (ie the one in /var) across to the destination.
For transient guests everything is working just fine.  For
persistent guests, this means that by default, they will
not have any config in /etc/libvirt/qemu after migration.
With a new enough libvirt, you can ask it to also persist
the configuration on the new host, using the --persistent
flag to 'virsh migrate'. You can also make it delete the
XML on the original host with  --undefinesource.

> 2.  I have been reading about KvmMigration using rgmanager,
> http://sources.redhat.com/cluster/wiki/KvmMigration, and here it
> implies that the xml files can be on gfs2 filesystems.  In particular:
>  Example vm using xmlfile:
> 
>   <vm name="mydomain" xmlfile="/mounts/gfs-guests/mydomain.xml" />
> 
> I assume that this would go in the cluster.conf file within the <rm> tags?

This kind of setup is fine. From libvirt's POV the guests are all
transient, since there is no persistent confi in /etc/libvirt/qemu.
The persistent configs are all in your seperate GFS2 cluster FS.
When ClusterSuite needs to start a guest, the XML is loaded off
GFS2 and given to libvirt at time of boot creating a transient
guest.

Any architecture with a centralized management server will typically
use transient guests, and ignore libvirt's persistent guest capability.
They will either have XML configs in some shared FS as is described
above, or may not store XML anywhere at all. eg RHEV-M/oVirt would 
store a description of the guest in a SQL database and auto-generate
XML only when needing to boot it

> Essentially, this is what I'm trying to get ironed out:
> 
> A.  I have 2 SAN arrays that I will use multipath on between my two
> hosts, so I would like to use GFS2 formatted partitions to store the
> vm disk images.
> B.  I would then have to have another partition that is GFS2 formatted
> to hold the vm xml files.  Would this partition also be on the SAN, or
> can I have the same mount point on both host servers (on local disks
> there), formatted to GFS2, and they would share with each other the
> xml files?

If you're keeping the master XML files outside of /etc/libvirt/qemu,
on some shared FS, they there's no restrictions, since they won't
be managed by libvirt at all. You could thus even put the XML files
at the same place as the disk images if you like.

Daniel
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