[Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts

[Kevin Wolf <kwolf@redhat.com>]

Am 11.09.2010 16:04, schrieb Anthony Liguori:
> This fixes a couple nasty problems relating to live migration.
> 
> 1) When dealing with shared storage with weak coherence (i.e. NFS), even if
>    we re-read, we may end up with undesired caching.  By delaying any reads
>    until we absolutely have to, we decrease the likelihood of any undesirable
>    caching.
> 
> 2) When dealing with copy-on-read, the local storage acts as a cache.  We need
>    to make sure to avoid any reads to avoid polluting the local cache.
> 
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

I think we should also delay even opening the image file at all to the
latest possible point to avoid that new problems of this kind are
introduced. Ideally, opening the image would be the very last thing we
do before telling the migration source that we're set and it should
close the images.

Even better would be to only open the image when the source has already
closed it (we could completely avoid the invalidation/reopen then), but
I think you were afraid that we might lose the image on both ends.

Kevin