On Wed, Apr 20, 2011 at 02:23:29PM -0400, Chris Mason wrote: > # fsync-tester > setting up random write file > done setting up random write file > starting fsync run > starting random io! > write time 0.0009s fsync time: 2.0142s > write time 128.9305s fsync time: 2.6046s > run done 2 fsyncs total, killing random writer > > In this case the 128s spent in write was on a single 4K overwrite on a > 4K file. I can't really reproduce this locally on XFS: setting up random write file done setting up random write file starting fsync run starting random io! write time: 0.0023s fsync time: 0.5949s write time: 0.0605s fsync time: 0.2339s write time: 0.0018s fsync time: 0.0179s write time: 0.0020s fsync time: 0.0201s write time: 0.0019s fsync time: 0.0176s write time: 0.0018s fsync time: 0.0209s write time: 0.0025s fsync time: 0.0197s write time: 0.0013s fsync time: 0.0183s write time: 0.0013s fsync time: 0.0217s write time: 0.0016s fsync time: 0.0158s write time: 0.0022s fsync time: 0.0240s write time: 0.0024s fsync time: 0.0190s write time: 0.0017s fsync time: 0.0205s write time: 0.0030s fsync time: 0.0688s write time: 0.0045s fsync time: 0.0193s write time: 0.0022s fsync time: 0.0356s But given that you are able to reproduce it, does the following patch help your latencies? Currently XFS actually does stop I/O when nr_to_write reaches zero, but only for non-blocking I/O. This behaviour was introduced in commit efceab1d563153a2b1a6e7d35376241a48126989 "xfs: handle negative wbc->nr_to_write during sync writeback" and works around issues in the generic writeback code. Index: linux-2.6/fs/xfs/linux-2.6/xfs_aops.c =================================================================== --- linux-2.6.orig/fs/xfs/linux-2.6/xfs_aops.c 2011-04-21 10:20:48.303550404 +0200 +++ linux-2.6/fs/xfs/linux-2.6/xfs_aops.c 2011-04-21 10:20:58.203496773 +0200 @@ -765,8 +765,7 @@ xfs_convert_page( SetPageUptodate(page); if (count) { - if (--wbc->nr_to_write <= 0 && - wbc->sync_mode == WB_SYNC_NONE) + if (--wbc->nr_to_write <= 0) done = 1; } xfs_start_page_writeback(page, !page_dirty, count); -- 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