From fc0561cefc04e7803c0f6501ca4f310a502f65b8 Mon Sep 17 00:00:00 2001 From: Dave Chinner Date: Tue, 3 Nov 2015 13:14:59 +1100 Subject: xfs: optimise away log forces on timestamp updates for fdatasync xfs: timestamp updates cause excessive fdatasync log traffic Sage Weil reported that a ceph test workload was writing to the log on every fdatasync during an overwrite workload. Event tracing showed that the only metadata modification being made was the timestamp updates during the write(2) syscall, but fdatasync(2) is supposed to ignore them. The key observation was that the transactions in the log all looked like this: INODE: #regs: 4 ino: 0x8b flags: 0x45 dsize: 32 And contained a flags field of 0x45 or 0x85, and had data and attribute forks following the inode core. This means that the timestamp updates were triggering dirty relogging of previously logged parts of the inode that hadn't yet been flushed back to disk. There are two parts to this problem. The first is that XFS relogs dirty regions in subsequent transactions, so it carries around the fields that have been dirtied since the last time the inode was written back to disk, not since the last time the inode was forced into the log. The second part is that on v5 filesystems, the inode change count update during inode dirtying also sets the XFS_ILOG_CORE flag, so on v5 filesystems this makes a timestamp update dirty the entire inode. As a result when fdatasync is run, it looks at the dirty fields in the inode, and sees more than just the timestamp flag, even though the only metadata change since the last fdatasync was just the timestamps. Hence we force the log on every subsequent fdatasync even though it is not needed. To fix this, add a new field to the inode log item that tracks changes since the last time fsync/fdatasync forced the log to flush the changes to the journal. This flag is updated when we dirty the inode, but we do it before updating the change count so it does not carry the "core dirty" flag from timestamp updates. The fields are zeroed when the inode is marked clean (due to writeback/freeing) or when an fsync/datasync forces the log. Hence if we only dirty the timestamps on the inode between fsync/fdatasync calls, the fdatasync will not trigger another log force. Over 100 runs of the test program: Ext4 baseline: runtime: 1.63s +/- 0.24s avg lat: 1.59ms +/- 0.24ms iops: ~2000 XFS, vanilla kernel: runtime: 2.45s +/- 0.18s avg lat: 2.39ms +/- 0.18ms log forces: ~400/s iops: ~1000 XFS, patched kernel: runtime: 1.49s +/- 0.26s avg lat: 1.46ms +/- 0.25ms log forces: ~30/s iops: ~1500 Reported-by: Sage Weil Signed-off-by: Dave Chinner Reviewed-by: Brian Foster Signed-off-by: Dave Chinner --- fs/xfs/xfs_file.c | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) (limited to 'fs/xfs/xfs_file.c') diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c index e78feb400e22..c94699cbc667 100644 --- a/fs/xfs/xfs_file.c +++ b/fs/xfs/xfs_file.c @@ -242,19 +242,30 @@ xfs_file_fsync( } /* - * All metadata updates are logged, which means that we just have - * to flush the log up to the latest LSN that touched the inode. + * All metadata updates are logged, which means that we just have to + * flush the log up to the latest LSN that touched the inode. If we have + * concurrent fsync/fdatasync() calls, we need them to all block on the + * log force before we clear the ili_fsync_fields field. This ensures + * that we don't get a racing sync operation that does not wait for the + * metadata to hit the journal before returning. If we race with + * clearing the ili_fsync_fields, then all that will happen is the log + * force will do nothing as the lsn will already be on disk. We can't + * race with setting ili_fsync_fields because that is done under + * XFS_ILOCK_EXCL, and that can't happen because we hold the lock shared + * until after the ili_fsync_fields is cleared. */ xfs_ilock(ip, XFS_ILOCK_SHARED); if (xfs_ipincount(ip)) { if (!datasync || - (ip->i_itemp->ili_fields & ~XFS_ILOG_TIMESTAMP)) + (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP)) lsn = ip->i_itemp->ili_last_lsn; } - xfs_iunlock(ip, XFS_ILOCK_SHARED); - if (lsn) + if (lsn) { error = _xfs_log_force_lsn(mp, lsn, XFS_LOG_SYNC, &log_flushed); + ip->i_itemp->ili_fsync_fields = 0; + } + xfs_iunlock(ip, XFS_ILOCK_SHARED); /* * If we only have a single device, and the log force about was -- cgit v1.2.3