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Date: Thu, 23 Dec 2010 02:34:46 +0000
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Subject: Re: [Qemu-devel] Asynchronously Mirroring a Virtual Machine
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: Tomer Margalit <tomermargalit@gmail.com>
Cc: scipioenterprises@yahoo.com, "Zaidenberg,
	Nezer" <Nezer.Zaidenberg@edfenergy.com>, qemu-devel@nongnu.org, Nezer Zaidenberg <nzaidenberg@mac.com>, Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>

On Wed, Dec 22, 2010 at 12:43 PM, Tomer Margalit
<tomermargalit@gmail.com> wrote:
> What I would like to add to it is the ability to asynchronously mirror a
> (QEMU) VM as well, so that if the primary site crashes, the VM can be
> restored (to the last stable point) immediately.
> Since I want it to be as general as possible, I will not rely on my mirror,
> but only on there being a virtual block device that is mirroring me (and
> that has consistent writes (that is, if write A succeeded, and afterwards
> write B succeeded, then if B is on the secondary site, then A must be there
> too)), and that there is some kind of marking mechanism (so that I can mark
> the last consistent state).
> The reason I want to do this using QEMU is that it has live migration -
> which is almost what I need.
> Right now my plan is to treat the memory as a file, and put it and all the
> vm images on the same mirroring block device.

Have you looked at Kemari for KVM (try searching qemu-devel)?  It
builds a fault tolerance mechanism on top of live migration.  Memory,
disk, and network changes are sent to a secondary machine which
passively tracks the state of the VM.  You can fail over to the last
completed transaction state.  Sounds similar to what you want to do.

Stefan