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Date: Wed, 6 Mar 2013 13:59:12 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130306125912.GF1954@stefanha-thinkpad.muc.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v4 0/6] Efficient VM backup for qemu
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To: Dietmar Maurer <dietmar@proxmox.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Wenchao Xia <xiawenc@linux.vnet.ibm.com>

On Mon, Feb 25, 2013 at 04:35:09PM +0000, Dietmar Maurer wrote:
> > >>> If we use nbd, how can we pass additional information to the other
> > >>> side, for example information about unallocated regions?
> > >>>
> > >>
> > >> You can either send trim commands, or just skip those regions and let
> > >> the other side figure it out.
> > >
> > > But we are lost if we want to transfer something else? I am currently
> > > not sure what we want to transfer, but I am sure we will find something in
> > future.
> > >
> > > Or can we simply add a header to each write with additional information?
> > 
> > Let's discuss it once we have a clearer picture of what that additional info would
> > be.
> 
> OK, just discovered such case. 
> 
> For example it would be nice to pass the size of the devices somehow?

That is not necessary since the management tool that starts the VMA
write process can pass this info (from 'query-block' QMP command):

  $ vma-writer --output backup-20130201.vma \
               --add-file path/to/vm-config.xml \
               --vmstate /tmp/backup-20130201.vmstate \
               --nbd /tmp/backup-20130201.nbd \
               --blockdev name=virtio-drive0,size=64424509440 \
               --blockdev name=virtio-drive1,size=128849018880

Note that this syntax uses Paolo's suggestions for UNIX domain sockets
and NBD named exports.

Stefan