[ovirt-devel] Default virt lock manager and Sanlock for Hosted Engine HA

[Daniel P. Berrange]

On Mon, Mar 23, 2015 at 12:00:12PM +0100, Sandro Bonazzola wrote:
> Hi,
> while working on a patch for hosted engine[1] it has been raised an objection about limiting the use of sanlock as lock manager to the Hosted Engine
> VM only.
> 
> "the ha should dynamically set libvirt if possible, or we need to also revert this part if disabled.
> also, if ever vdsm will require to configure this file we will have a race which is not healthy.
> it seems that you can define domain and per domain setting and assign your vm to that domain instead of effecting entire host."
> 
> But looking at Fedora Virt Lock Manager feature[2] looks like we should configure libvirt for using sanlock by default and not only for Hosted Engine HA.
> 
> If we decide that we should keep the default lockd manager instead of the sanlock one I wpuld like to ask virt team which is the best way for setting
> sanlock only for the hosted engine VM.
> I've seen I can change the libvirt xml using a vdsm hook and pass command line options to qemu[3] but I can't see an option to pass to qemu-kvm for that.
> 
> There was also an ongoing discussion about locking per disk / per vm on the storage team part, so maybe this is not the best way to get this done
> after all.
> 
> What do you think / suggest?

Libvirt has a pluggable lock manager engine, of which there are two impls,
one using virtlockd and one using sanlock. Libvirt does not currently
enable any lock manager plugin by default, but our intention is that a
future release will in fact enable virtlockd by default.

The reason for this is that virtlockd to be a more broadly usable and
flexible solution, in particular we can make it essentially zero config
for admin. While libvirt did attempt to provide a zero config mode for
the sanlock plugin, by creating lease files on shared filesystems, the
sanlock maintainers explicitly recommend against this usage scenario.
It also has significant I/O performance and storage implications, and
so is not really satisfactory for deployment without knowledgable
admins or an mgmt app such as VDSM to actively manage it usage.

As such our recommendation is that sanlock should be enabled only when
explicitly required on the host by the mgmtm application being deployed.
ie only when oVirt/VSM is enabled on the host.

Regards,
Daniel
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