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Re: [Libguestfs] Point-in-time snapshots (was: Re: Inspection of disk snapshots)



On Fri, Mar 27, 2015 at 03:21:25PM +0000, Stefan Hajnoczi wrote:
> On Fri, Mar 27, 2015 at 12:31:41PM +0000, Richard W.M. Jones wrote:
> > On Fri, Mar 27, 2015 at 12:15:34PM +0000, Stefan Hajnoczi wrote:
> > > On Thu, Mar 26, 2015 at 12:02:27AM +0100, Kashyap Chamarthy wrote:
> > > > So, something like?
> > > > 
> > > >   . . .
> > > >   { 'execute': 'drive-backup', 'arguments':
> > > >         { 'device': 'drive-virtio-disk1', 'sync': 'full', 'target':
> > > >         'nbd://localhost', 'mode': 'absolute-paths', 'format': 'qcow2' } 
> > > >   . . .
> > > > 
> > > > Same question as yours, what is the NBD server going to run?
> > > > 
> > > > 
> > > > My only NBD testing so far has been with w/ NBD over Unix socket or
> > > > over TCP[**].
> > > 
> > > For 'sync': 'full' mode qemu-nbd or nbd-server can be used as the
> > > server.  You probably don't want 'format': 'qcow2', just raw data over
> > > NBD.  That way the NBD server can implement whatever storage backend it
> > > likes (raw, qcow2, something else).
> > > 
> > > For incremental backup the NBD server would either be a qemu-nbd
> > > instance with a qcow2 image and backing file:
> > > 
> > >   full-backup.img <- incremental-1.qcow2 <- incremental-2.qcow2
> > > 
> > > Or the NBD server would be a custom backup application that does
> > > something smart with the incoming NBD WRITE requests.
> > 
> > What I care about is connecting libguestfs to qemu and reading a
> > snapshot at some point in time, even though the guest is still writing
> > away to its disks.  Is this possible with drive-backup (or otherwise)?
> 
> Yes, that is what drive-backup does.
> 
> New writes coming from the guest are held up until the old data has been
> written to the NBD target.
> 
> That way you get a point-in-time snapshot while the guest continues
> running.

I understand how that can work for backups, where you want to copy
a whole disk consistently.

But libguestfs doesn't want to do a backup, nor get a copy of the
whole disk, it just wants to access a scattering of blocks (maybe a
few hundred) but at a single point in time, in as lightweight a manner
as possible.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v


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