Re: [libvirt] block-backup API for libvirt
On Fri, Nov 08, 2013 at 05:13:00PM -0700, Eric Blake wrote:
On 11/08/2013 01:56 PM, Ian Main wrote:
>
> In qemu there will soon be drive-backup and block-backup commands that
> allow you to create backups or point-in-time snapshots of VM images.
> The original feature that drove this was to allow 'image fleecing',
> which involves creating a light weight mechanism to take a point in time
> snapshot of the running VM and access that remotely.
>
> You can get a good idea of the API from this post:
>
> http://lists.gnu.org/archive/html/qemu-devel/2013-10/msg02266.html
>
> I would like to see this available via libvirt and wanted to start
> working on a suitable API. I'm curious if anyone has any input on how
> they would like to see this done? While I have done some work with and
> on libvirt I'm no expert on the design philosophy behind the APIs so
> some direction here before I get coding would be much appreciated.
>
>>From what I can tell a volume based API that creates 'backups' and does
> the NBD export call could work but I'd like to hear if that's
> reasonable.
It seems to me that a new API virDomainBlockBackup(domain, disk,
destination, bandwidth, flags) is probably the right approach, as well
as enhancing the existing virDomainBlockJob* to report job status and/or
abort the job early.
OK that sounds good. How would it look if you wanted to NBD export the
disk? Would that require a special desitination uri of some sort, or?
Also in order to do that we need to run several commands against qemu
including building a temporary image to store the original blocks. Is
there an example command which already uses temporary storage that I
could look at?
For that matter, we need to overhaul the job support. Right now,
virDomainBlockJob* assumes you can only have one outstanding job at a
time, which interferes with the notion of running fleecing from two
points in time. Similarly, there can only be one virDomainGetJobInfo
active. What we really need is a notion of a job id, a way to query the
job id most recently created by any existing job-related API, and a way
to query job status based on id rather than only the most-recently
started job.
OK I can look at this too. I need to focus on getting the qemu patches
in and then I'll start tackling all this.
Ian
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org