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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection
Date: Thu, 23 May 2013 09:50:32 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

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.


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