On Tue, Aug 03, 2010 at 04:41:14PM -0700, Mingming Cao wrote: > On Wed, 2010-08-04 at 08:56 +1000, Dave Chinner wrote: > > On Tue, Aug 03, 2010 at 10:34:25AM -0700, Mingming Cao wrote: > > > On Fri, 2010-07-30 at 14:53 +1000, Dave Chinner wrote: > > > > On Thu, Jul 29, 2010 at 08:53:24PM -0600, Matthew Wilcox wrote: > > > > > On Fri, Jul 30, 2010 at 08:45:16AM +1000, Dave Chinner wrote: > > > > > > If we get two unaligned direct IO's to the same filesystem block > > > > > > that is marked as a new allocation (i.e. buffer_new), then both IOs will > > > > > > zero the portion of the block they are not writing data to. As a > > > > > > result, when the IOs complete there will be a portion of the block > > > > > > that contains zeros from the last IO to complete rather than the > > > > > > data that should be there. > > .... > > > > I don't want any direct IO for XFS to go through the page cache - > > > > unaligned or not. using the page cache for the unaligned blocks > > > > would also be much worse for performance that this method because it > > > > turns unaligned direct IO into 3 IOs - the unaligned head block, the > > > > aligned body and the unaligned tail block. It would also be a > > > > performance hit you take on every single dio, whereas this way the > > > > hit is only taken when an overlap is detected. > > > > > > Does this problem also possible for DIO and non AIO case? (Ext4 case > > > this only happy with AIO+DIO+unaligned). If not, could we simply force > > > unaligned AIO+DIO to be synchronous? Still direct IO... > > > > There is nothing specific to AIO about this bug. XFS (at least) > > allows concurrent DIO writes to the same inode regardless of whether > > they are dispatched via AIO or multiple separate threads and so the > > race condition exists outside just the AIO context... > > > > Okay..yeah ext4 prevent direct IO write to the same inode from multiple > threads, so this is not a issue for non-aio case. > > How does XFS serialize direct IO (aligned) to the same file offset(or > overlap) from multiple threads? It doesn't. The 1996 usenix paper describes it well: http://oss.sgi.com/projects/xfs/papers/xfs_usenix/index.html See section 6.2 "Performing File I/O", sepcifically the sections on "Using Direct I/O" and "Using Multiple Processes". Quote: "When using direct I/O, multiple readers and writers can all access the file simultaneously. Currently, when using direct I/O and multiple writers, we place the burden of serializing writes to the same region of the file on the application." This is still true 15 years later.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
