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Date: Mon, 18 Mar 2013 11:08:56 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] qmp interface for save vmstate to image
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>, Dietmar Maurer <dietmar@proxmox.com>, Wenchao Xia <xiawenc@linux.vnet.ibm.com>

Il 18/03/2013 10:04, Kevin Wolf ha scritto:
> Oh, seems I misread this then. I thought this was about internal live
> snapshots, which is a feature that I consider really useful. I'm not so
> sure if saving the VM state as the disk contents of a qcow2 image is
> really helpful.
> 
> If zero clusters help a lot, then there's clearly something to improve
> in the migration protocol, because it shouldn't send so many zeros in
> the first place.

Zero pages are sent as a single 9-byte entry (64 bits for the address
and flags, 8 for the zero).

I don't expect the migration stream to have a single zero cluster, since
every page is prefixed by the 64 bits for the address and flags.
Furthermore, the RAM data would be horribly unaligned because of this.
15-20% sectors or so would be read twice, since reading each page (4104
bytes including the address and flags) would span 10 sectors (5120 bytes).

Paolo