On Wed, Apr 18, 2012 at 12:07:17PM -0400, Ted Ts'o wrote: > On Wed, Apr 18, 2012 at 01:02:08PM +1000, Dave Chinner wrote: > > In actual fact, on my 12 disk RAID0 array, XFS is faster with > > unwritten extents *enabled* than when hacked to turn them off. > > Can you explain why this is the case? It seems... counterintuitive. I'll admit that I was slightly surprised, too. > The only explanation I can think of is that your code paths when > unwritten extents are disabled haven't been optimized, When unwritten extents are disabled, the IO takes the normal overwrite/extending write path, which is as optimised as the unwritten path because it is more commonly used. > in which case > the comparison between using and not using unwritten extents might not > be valid. It's and apples to apples comparison. The code paths are identical on IO submission, the only difference is the IO completion path. In the case of unwritten extents, it has to do a transaction to convert the extents and update the file size. In the case of the stale extent, it has to modify the file size because the IO is beyond EOF. My point, however, is that despite the complexity of unwritten extent conversion it is not noticably more expensive than a file size update transaction on XFS. Hence the reasoning for exposing stale data due to unwritten conversion has too much overhead to be useful is demonstratably false. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
