Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection

[Stefan Hajnoczi]

On Wed, May 22, 2013 at 11:46:15PM +0200, Paolo Bonzini wrote:
> Il 22/05/2013 22:47, Richard W.M. Jones ha scritto:
> >> > 
> >> > I meant if there was interest in reading from a disk that isn't fully
> >> > synchronized
> >> > (yet) to the original disk (it might have old blocks).  Or would you only
> >> > want to
> >> > connect once a (complete) snapshot is available (synchronized completely 
> >> > to
> >> > some point-in.
> > IIUC a disk which wasn't fully synchronized wouldn't necessarily be
> > interpretable by libguestfs, so I guess we would need the complete
> > snapshot.
> 
> In the case of point-in-time backups (Stefan's block-backup) the plan is
> to have the snapshot complete from the beginning.

The way it will work is that the drive-backup target is a qcow2 image
with the guest's disk as its backing file.  When the guest writes to the
disk, drive-backup copies the original data to the qcow2 image.

The qcow2 image is exported over NBD so a client can connect to access
the read-only point-in-time snapshot.  It is not necessary to populate
the qcow2 file since it uses the guest disk as its backing file - all
reads to unpopulated clusters go to the backing file.

Stefan