Re: [Qemu-devel] [PATCH v2 0/2] block/file-posix: allow -drive cache.direct=off live migration

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Apr 27, 2018 at 05:23:10PM +0100, Stefan Hajnoczi wrote:
> v2:
>  * Add comment on !__linux__ situation [Fam]
>  * Add file-posix.c x-check-cache-dropped=on|off option [DaveG, Kevin]
> 
> file-posix.c only supports shared storage live migration with -drive
> cache.direct=off due to cache consistency issues.  There are two main shared
> storage configurations: files on NFS and host block devices on SAN LUNs.
> 
> The problem is that QEMU starts on the destination host before the source host
> has written everything out to the disk.  The page cache on the destination host
> may contain stale data read when QEMU opened the image file (before migration
> handover).  Using O_DIRECT avoids this problem but prevents users from taking
> advantage of the host page cache.
> 
> Although cache=none is the recommended setting for virtualization use cases,
> there are scenarios where cache=writeback makes sense.  If the guest has much
> less RAM than the host or many guests share the same backing file, then the
> host page cache can significantly improve disk I/O performance.
> 
> This patch series implements .bdrv_co_invalidate_cache() for block/file-posix.c
> on Linux so that shared storage live migration works.  I have sent it as an RFC
> because cache consistency is not binary, there are corner cases which I've
> described in the actual patch, and this may require more discussion.
> 
> Regarding NFS, QEMU relies on O_DIRECT rather than the close-to-open
> consistency model (see nfs(5)), which is the basic guarantee provided by NFS.
> After this patch cache consistency is no longer provided by O_DIRECT.
> 
> This patch series relies on fdatasync(2) (source) +
> posix_fadvise(POSIX_FADV_DONTNEED) (destination) instead.  I believe it is safe
> for both NFS and SAN LUNs.  Maybe we should use fsync(2) instead of
> fdatasync(2) so that NFS has up-to-date inode metadata?
> 
> Stefan Hajnoczi (2):
>   block/file-posix: implement bdrv_co_invalidate_cache() on Linux
>   block/file-posix: add x-check-page-cache=on|off option
> 
>  qapi/block-core.json |   7 ++-
>  block/file-posix.c   | 146 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 150 insertions(+), 3 deletions(-)
> 
> -- 
> 2.14.3
> 

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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