[libvirt] Question about fsfreeze/fsthaw API

[Daniel P. Berrange]

On Thu, Nov 07, 2013 at 09:39:01AM +0100, Michal Privoznik wrote:
> On 07.11.2013 01:31, Tomoki Sekiyama wrote:
> > Hi all,
> > 
> > Is there any plans to add APIs to execute fsfreeze/fsthaw in qemu guests?
> > (something like virDomainFSFreeze(domain,timeout,flags) and
> >                 virDomainFSThaw(domain,timeout,flags))
> > 
> > These would be useful in the case a guest has a disk device with its own
> > snapshot feature, such as cinder volumes in OpenStack configuration.
> > In such cases, libvirt clients want to issue fsfreeze/fsthaw before/after
> > taking the disk snapshot.
> > 
> > Currently we can execute them using virDomainQemuAgentCommand(). (e.g.
> > virsh qemu-agent-command dom '{"execute":"guest-fsfreeze-freeze"}' )
> > 
> > However, this is exposing internal implementation too much. And it
> > cannot leverage future implementation for the other hypervisors.
> > So it would be nice if we have well-defined API for fsfreeze/fsthaw.
> > 
> > If there is no plan for these API and this is acceptable, I will try to
> > implement this.
> > Any comments are welcome.
> 
> I am not aware of somebody wanting these APIs exposed hence I don't know
> if somebody is working on it. But since there is a clear usage scenario
> I'd say go for it.

We already support this QEMU agent commands in libvirt. We decided not
to expose them explicitly as APIs, but instead wire them upto the impl
of the disk snapshot APIs when VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE is
used.

Furthermore, OpenStack already has support for using this flag when
doing snapshots, so I don't think we need any more work here.

Daniel
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