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
signature.asc
Description: PGP signature
