[Qemu-devel] QEMU Live Snapshots / Commiting

[Robert P <robp236@gmail.com>]

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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.

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.

Thanks.
Robert

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