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Date: Tue, 4 Mar 2014 09:54:56 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20140304085456.GD25676@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [RFC]VM live snapshot proposal
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To: "Huangpeng (Peter)" <peter.huangpeng@huawei.com>
Cc: "kwolf@redhat.com" <kwolf@redhat.com>, Pavel Hrdina <phrdina@redhat.com>, Zhanghailiang <zhang.zhanghailiang@huawei.com>, KVM devel mailing list <kvm@vger.kernel.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>, Wenchao Xia <xiawenc@linux.vnet.ibm.com>

On Tue, Mar 04, 2014 at 01:02:44AM +0000, Huangpeng (Peter) wrote:
> > But back to the options:
> > 
> > If the host has enough free memory to fork QEMU, a small helper process can
> > be used to save the copy-on-write memory snapshot (thanks to fork(2)
> > semantics).  The hard part about the fork(2) approach is that QEMU isn't
> > really designed to fork, so work is necessary to reach a quiescent state for the
> > child process.
> > 
> > If there is not enough memory to fork, then a synchronous approach to
> > catching guest memory writes is needed.  I'm not sure if a good mechanism
> > for that exists but the simplest would be mprotect(2) and a signal handler
> > (which will make the guest run very slowly).
> > 
> > Stefan
> 
> In real production environment, memory over-commit or use as much memory as
> possible may be the normal case, so the fork semantics cannot meet the needs.  

Yes, I think you're right.  The fork approach only works in the easy
case where there is plenty of free host memory.

> Is there any other proposals to implement vm-snapshot?

See the discussion by Paolo and Andrea about post-copy migration, which
adds kernel memory management features for tracking userspace page
faults.  Perhaps you can use that infrastructure to trap guest writes.

Stefan