[libvirt] image locking?

[Guido Günther]

On Tue, Oct 21, 2008 at 12:16:01PM -0700, Itamar Heim wrote:
> While this might work for SBC (although most enterprises have the
> datacenter on remote sites as well, so not always that easy).  I don't
> think the solution is viable for CBC though (I am not sure CBC would
> use iSCSI, probably NFS is a more relevant option, but the leased
> locking is required there as well, just as a collaborative effort to
> notate to the non-responding node to stop writing to the image).
What about using dlm? This gives fencing for free. See my other post
- allowing libvirt to use cluster wide storage locks would solve your
problem?
 -- Guido

> 
> -----Original Message-----
> From: Perry Myers [mailto:pmyers at redhat.com] 
> Sent: Tuesday, October 21, 2008 21:09 PM
> To: Itamar Heim
> Cc: libvir-list at redhat.com
> Subject: Re: [libvirt] image locking?
> 
> Itamar Heim wrote:
> > Hi Perry,
> > 
> > The problem is with unreachable hosts which are locking the image
> > forever.
> > 
> > When fencing can't be used, there is no way for the management to
> > "release" the image, since it can't verify the host stopped using the
> > image. A leased lock mechanism, while not providing 100% prevention,
> > does allow a collaborative effort to allow releasing the image after
> > the lock expired, by having the nodes check that they still own the
> > lease and stop writing to the images.
> 
> If you have an unreachable host that is locking the image forever, you
> walk into the datacenter and pull the plug.  Once that is done, you can
> use the oVirt Server interface to undefine the vm and release the storage
> volume.  So it can be done without hw fencing, it just involves manual
> administrator action.  Not ideal, but it works :)
> 
> > It would have been much better if image access could have been enforced
> > at storage level, but that is much more complex (and not relevant for
> > images under LVM for example)
> 
> Agreed.  We're using the above procedure (pull the plug or hw fencing) 
> until a better mechanism is created.
> 
> Perry
> 
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