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

[Juan Quintela <quintela@redhat.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.