[libvirt] image locking?

[Itamar Heim]

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).

-----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