On Mon, May 11, 2009 at 12:47:58PM -0500, Anthony Liguori wrote: > But how do we define the data integrity guarantees to the user of > cache=writeback+fsync? It seems to require a rather detailed knowledge > of Linux's use of T_FLUSH operations. It does work the same as for disks with writeback caches. If a barrier request completes the filesystem can assume all previous I/O on this disk has finished before the barrier request, and the barrier request has finished after these without any later request being finished before it. If a cache flush is issued the cache as of that point in time is flushed (used for fsync). > Right now, it's fairly easy to understand. cache=none and > cache=writethrough guarantee that all write operations that the guest > thinks have completed are completed. cache=writeback provides no such > guarantee. As said above with barriers it indeed does. > cache=writeback+fsync would guarantee that only operations that include > a T_FLUSH are present on disk which currently includes fsyncs but does > not include O_DIRECT writes. I guess whether O_SYNC does a T_FLUSH also > has to be determined. O_SYNC and O_DIRECT do it at least on XFS due to updating metadata after the write. I can't vouch for implementation details on other filesystems. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
