[libvirt-users] VM locking

[Daniel P. Berrange]

On Mon, Aug 31, 2015 at 08:01:58PM +0000, Prof. Dr. Michael Schefczyk wrote:
> Dear All,
> 
> I am trying to use VM (disk) locking on a two node Centos 7 KVM cluster. Unfortunately, I am not successful.
> 
> Using virtlockd (https://libvirt.org/locking-lockd.html), I get each
> host to write the zero length file with a hashed filename to the shared
> folder specified. Regardless of which host I start a VM (domain) on,
> they do produce the identical filename per VM. What does not work,
> however, is to prevent the second host to start the VM already running
> on the first VM.

Ok, so you've configured the locks to happen on a shared filesystem
which sounds correct.

> My system is current Centos 7 using a gluster storage setup like suggested
> for oVirt (https://community.redhat.com/blog/2014/10/up-and-running-with-ovirt-3-5/)
> based on the oVirt 3.5 repo but without an engine. I do this, because I
> want to retain VM files in qcow2 format for live backups and with human readable
> names. What does work then is, e.g., live migration. A locking mechanism to
> prevent starting a VM twice would be good.

Ok, so you are using gluster for your qcow2 files.  

> Please note that this configuration - both according to redhat and
> to my own trial and error - requires Lock=False in /etc/nfsmount.conf.
> Is there a connection with my findings? My issues occur regardless
> of the files being in NFS or Gluster folders. KVM must load the
> Gluster storage indirectly via NFS rather than straight via Gluster,
> as Gluster storage does not seem to fully work, at least in virt-manager.

I'm a little confused about whether the virtlockd lock directory is
stored on NFS or on GlusterFS at this point.

If using NFS though, the lock=false setting will definitely break
virtlockd. When you set lock=false, it means that any fcntl() locks
applications acquire are only scoped to the local host - no other
NFS clients will see them, which matches the behaviour you describe.

Regards,
Daniel
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