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[Qemu-devel] Re: [PATCH 1/3] block: allow migration to work with image f

From: Juan Quintela
Subject: [Qemu-devel] Re: [PATCH 1/3] block: allow migration to work with image files
Date: Wed, 15 Sep 2010 17:53:13 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux)

Anthony Liguori <address@hidden> 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 <address@hidden>

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.

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