On Wed, Oct 21, 2015 at 01:59:03PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> 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 subsequnet transactions, so it carries around the
subsequent
> 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 <sage@xxxxxxxxxx>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/xfs_file.c | 4 +++-
> fs/xfs/xfs_inode.c | 2 ++
> fs/xfs/xfs_inode_item.c | 1 +
> fs/xfs/xfs_inode_item.h | 1 +
> fs/xfs/xfs_trans_inode.c | 9 +++++++++
> 5 files changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 0045b0a..cb4204d 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -248,8 +248,10 @@ xfs_file_fsync(
> 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;
> + ip->i_itemp->ili_fsync_fields = 0;
> + }
Ok, so we check what's been logged since the last fsync that forced the
log. If anything other than the timestamp has been logged, we force the
log and clear the fields. Seems like a reasonable optimization to me.
One question... is it safe to clear the ili_fsync fields here if we have
parallel fsync()/fdatasync() calls coming in? This is under the shared
ilock, so assume that one fsync() comes in and finds non-timestamp
changes to flush. It grabs the lsn, clears the flags and calls the log
force. If an fdatasync() comes in before the log force completes,
shouldn't it wait?
Also, is it me or are we sending an unconditional flush in the hunk
following the log force call in xfs_file_fsync() (even if we've skipped
the log force)?
Brian
> }
> xfs_iunlock(ip, XFS_ILOCK_SHARED);
>
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 4c52e3f..144baab 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -2379,6 +2379,7 @@ retry:
>
> iip->ili_last_fields = iip->ili_fields;
> iip->ili_fields = 0;
> + iip->ili_fsync_fields = 0;
> iip->ili_logged = 1;
> xfs_trans_ail_copy_lsn(mp->m_ail, &iip->ili_flush_lsn,
> &iip->ili_item.li_lsn);
> @@ -3574,6 +3575,7 @@ xfs_iflush_int(
> */
> iip->ili_last_fields = iip->ili_fields;
> iip->ili_fields = 0;
> + iip->ili_fsync_fields = 0;
> iip->ili_logged = 1;
>
> xfs_trans_ail_copy_lsn(mp->m_ail, &iip->ili_flush_lsn,
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index 62bd80f..d14b12b 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -719,6 +719,7 @@ xfs_iflush_abort(
> * attempted.
> */
> iip->ili_fields = 0;
> + iip->ili_fsync_fields = 0;
> }
> /*
> * Release the inode's flush lock since we're done with it.
> diff --git a/fs/xfs/xfs_inode_item.h b/fs/xfs/xfs_inode_item.h
> index 488d812..4c7722e 100644
> --- a/fs/xfs/xfs_inode_item.h
> +++ b/fs/xfs/xfs_inode_item.h
> @@ -34,6 +34,7 @@ typedef struct xfs_inode_log_item {
> unsigned short ili_logged; /* flushed logged data */
> unsigned int ili_last_fields; /* fields when flushed */
> unsigned int ili_fields; /* fields to be logged */
> + unsigned int ili_fsync_fields; /* logged since last fsync */
> } xfs_inode_log_item_t;
>
> static inline int xfs_inode_clean(xfs_inode_t *ip)
> diff --git a/fs/xfs/xfs_trans_inode.c b/fs/xfs/xfs_trans_inode.c
> index 17280cd..b97f1df 100644
> --- a/fs/xfs/xfs_trans_inode.c
> +++ b/fs/xfs/xfs_trans_inode.c
> @@ -108,6 +108,15 @@ xfs_trans_log_inode(
> ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
>
> /*
> + * Record the specific change for fdatasync optimisation. This
> + * allows fdatasync to skip log forces for inodes that are only
> + * timestamp dirty. We do this before the change count so that
> + * the core being logged in this case does not impact on fdatasync
> + * behaviour.
> + */
> + ip->i_itemp->ili_fsync_fields |= flags;
> +
> + /*
> * First time we log the inode in a transaction, bump the inode change
> * counter if it is configured for this to occur. We don't use
> * inode_inc_version() because there is no need for extra locking around
> --
> 2.5.0
>
> _______________________________________________
> xfs mailing list
> xfs@xxxxxxxxxxx
> http://oss.sgi.com/mailman/listinfo/xfs
_______________________________________________
xfs mailing list
xfs@xxxxxxxxxxx
http://oss.sgi.com/mailman/listinfo/xfs
