Am 08.05.2018 um 12:32 hat Stefan Hajnoczi geschrieben: > 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(-) > > Kevin: Are you happy with this series?
Yes, I think I am. I'm still kind of concerned about misleading people into believing that cache=writeback + live migration is generally safe when it's only in special cases, but that's not really a concern about the code, but about how we communicate the feature. Kevin
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