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Re: [Qemu-devel] [PATCH 0/17 v3] Localhost migration with side channel f

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH 0/17 v3] Localhost migration with side channel for ram
Date: Thu, 21 Nov 2013 10:19:34 +0000
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Nov 21, 2013 at 05:11:23PM +0800, Lei Li wrote:
> This patch series tries to introduce a mechanism using side
> channel pipe for RAM via SCM_RIGHTS with unix domain socket
> protocol migration.
> This side channel is used for the page flipping by vmsplice,
> which is the internal mechanism for localhost migration that
> we are trying to add to QEMU. The backgroud info and previous
> patch series for reference,
> Localhost migration
> http://lists.nongnu.org/archive/html/qemu-devel/2013-08/msg02916.html
> migration: Introduce side channel for RAM
> http://lists.gnu.org/archive/html/qemu-devel/2013-09/msg04043.html
> I have picked patches from the localhost migration series and rebased 
> it on the series of side channel, now it is a complete series that
> passed the basic test.
> Please let me know if there is anything needs to be fixed or improved.
> Your suggestions and comments are very welcome, and thanks to Paolo
> for his continued review and useful suggestions.

In discussions about supporting this for libvirt, we were told that
when this localhost migration fails, you cannot re-start the guest
on the original source QEMU.

If this is true, this implementation is not satisfactory IMHO. One
of the main motivations of this feature is to allow for in-place
live upgrades of QEMU binaries, for people who can't tolerate the
downtime of restarting their guests, and whom don't have a spare
host to migrate them to. 

If people are using this because they can't tolerate any downtime
of the guest, then we need to be able to fully deal with failure to
complete migration by switching back to the original QEMU process,
as we can do with normal non-localhost migration.

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