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Date: Fri, 30 Sep 2011 09:16:27 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: Re: [Qemu-devel] QEMU Live Snapshots / Commiting
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To: Robert P <robp236@gmail.com>
Cc: qemu-devel@nongnu.org

On Thu, Sep 29, 2011 at 09:07:19PM +0200, Robert P wrote:
> Hello,
> 
> I still have a problem with the "Live Snapshot" feature of QEMU .... and
> before migrating to XEN, VMware or something similare, a quick post here:
> 
> OS: Ubuntu Natty 64bit
> 
> First, i'm starting my KVM Machine with an image like this:
> qemu-img create -f qcow2 -o backing_file=<NameOfBaseImage> <Snapshotname>
> 
> If i stop the KVM Machine later, and i commit <Snapshotname> into
> <NameOfBaseImage>, all the new changes are in the <NameOfBaseImage>.
> That would be ok.
> 
> ---
> 
> The Problem:
> 
> Actually i'm trying to create "live snapshots" periodically  while the
> machine is running, like this (host2Qemu is just a special function of mine
> (it works), to send a string to qemu-monitor).
> 
>                 host2Qemu "cont"
>                 host2Qemu "guest-agent-fsfreeze"
>                 host2Qemu "stop"
> 
>                 host2Qemu "info block"
>                 host2Qemu "snapshot_blkdev ide0-hd0 <Snapshot1 (example)>
> qcow2"
> 
>                 host2Qemu "cont"
>                 host2Qemu "guest-agent-fsthaw"
> 
> My idea is, to commit them one by one afterwards, when the KVM Machine is
> down into the BaseImage.
> 
> So, the Snapshots are beeing written, and everytime i call that function new
> data is beeing written to the new "alllocated" snapshot.
> BUT, committing of that live-snapshots fails, and i've no idea why ?!
> 
> I would commit it like that:
>  qemu-img commit -f qcow2 <Snapshot, with KVM was started first>
> qemu-img commit -f qcow2 <Snapshot1, newer>
> qemu-img commit -f qcow2 <Snapshot1, more new>
> ...
> and so on.
> 
> So in that constellation, only changes from the Snapshot, with KVM was
> started are in the Base-Image.

I'm not 100% clear that you know qemu-img commit cannot be used while
QEMU is running and has the disk image file open.  It's simply not safe
to modify the image file with qemu-img while QEMU is running.

> And another question: I have a Windows XP Guest also in KVM, but the write
> performance into the (qcow2 Image) (scp, rsync, e.g) Guest seems to be
> pretty poor - Are there any hints or is there a special Parameter to avoid
> this?
> The write Performance to a Linux Guest with a similar configuration and also
> qcow2 seems to very close to the "native" Performance.

Are you using the virtio drivers for Windows?

Stefan