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[Qemu-devel] QEMU Live Snapshots / Commiting

From: Robert P
Subject: [Qemu-devel] QEMU Live Snapshots / Commiting
Date: Thu, 29 Sep 2011 21:07:19 +0200


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.

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.


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