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From: Juan Quintela <quintela@redhat.com>
In-Reply-To: <1284213896-12705-2-git-send-email-aliguori@us.ibm.com> (Anthony
	Liguori's message of "Sat, 11 Sep 2010 09:04:54 -0500")
References: <1284213896-12705-1-git-send-email-aliguori@us.ibm.com>
	<1284213896-12705-2-git-send-email-aliguori@us.ibm.com>
Date: Wed, 15 Sep 2010 17:53:13 +0200
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Subject: [Qemu-devel] Re: [PATCH 1/3] block: allow migration to work with
	image files
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To: Anthony Liguori <aliguori@us.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Anthony Liguori <aliguori@us.ibm.com> wrote:
> Image files have two types of data: immutable data that describes things like
> image size, backing files, etc. and mutable data that includes offset and
> reference count tables.
>
> Today, image formats aggressively cache mutable data to improve performance.  In
> some cases, this happens before a guest even starts.  When dealing with live
> migration, since a file is open on two machines, the caching of meta data can
> lead to data corruption.
>
> This patch addresses this by introducing a mechanism to invalidate any cached
> mutable data a block driver may have which is then used by the live migration
> code.
>
> NB, this still requires coherent shared storage.  Addressing migration without
> coherent shared storage (i.e. NFS) requires additional work.
>
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

For the NFS case, we just need 2 different case:
- outgoing: nothing to do (generic code already do an fsync())
- incoming: we need to reopen the image.

For the rest, I agree.  Once told that, I can implement my changes here.

Later, Juan.