[PATCH] xfs: optimise away log forces on timestamp updates for fdatasync

[PATCH] xfs: optimise away log forces on timestamp updates for fdatasync

[Dave Chinner]

From: Dave Chinner <dchinner@redhat.com>

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
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@redhat.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 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;
+		}
 	}
 	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

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Re: [PATCH] xfs: optimise away log forces on timestamp updates for fdatasync

[Brian Foster]

On Wed, Oct 21, 2015 at 01:59:03PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> 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@redhat.com>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  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
> 
> _______________________________________________
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> xfs@oss.sgi.com
> http://oss.sgi.com/mailman/listinfo/xfs

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Re: [PATCH] xfs: optimise away log forces on timestamp updates for fdatasync

[Dave Chinner]

On Thu, Oct 22, 2015 at 01:36:19PM -0400, Brian Foster wrote:
> On Wed, Oct 21, 2015 at 01:59:03PM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > xfs: timestamp updates cause excessive fdatasync log traffic
....
> > +++ 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?

Probably, but the only way to do that is to run a log force on that
same lsn. Actually, it is safe to do that log force while holding
the XFS_ILOCK (xfs_trans_commit() does that for synchronous
transactions), so we should simply be able to do:

	xfs_ilock(ip, XFS_ILOCK_SHARED);
	if (xfs_ipincount(ip)) {
		if (!datasync ||
		    (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
			lsn = ip->i_itemp->ili_last_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);


> 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)?

The flush is needed - fdatasync needs to guarantee the data is
on stable storage even if no metadata needs to be written to the
journal.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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Re: [PATCH] xfs: optimise away log forces on timestamp updates for fdatasync

[Brian Foster]

On Mon, Oct 26, 2015 at 04:07:20PM +1100, Dave Chinner wrote:
> On Thu, Oct 22, 2015 at 01:36:19PM -0400, Brian Foster wrote:
> > On Wed, Oct 21, 2015 at 01:59:03PM +1100, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > > 
> > > xfs: timestamp updates cause excessive fdatasync log traffic
> ....
> > > +++ 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?
> 
> Probably, but the only way to do that is to run a log force on that
> same lsn. Actually, it is safe to do that log force while holding
> the XFS_ILOCK (xfs_trans_commit() does that for synchronous
> transactions), so we should simply be able to do:
> 

Yeah, a parallel fsync on a single inode situation is probably not the
most likely thing. Thinking about it further, the case where multiple
inodes happen to have the same lsn might be more likely and takes the
shared ilock out of the equation.

> 	xfs_ilock(ip, XFS_ILOCK_SHARED);
> 	if (xfs_ipincount(ip)) {
> 		if (!datasync ||
> 		    (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
> 			lsn = ip->i_itemp->ili_last_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);
> 

This looks better to me. We send the caller through the log force
infrastructure until we know the log has been forced.

> 
> > 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)?
> 
> The flush is needed - fdatasync needs to guarantee the data is
> on stable storage even if no metadata needs to be written to the
> journal.
> 

Ok. Well it's too bad we don't get any feedback about what was written
from the filemap_write_and_wait_range() call. As it is, we send a flush
even if there's nothing to write back. I suppose that's also not a major
problem since an fsync() caller implies something was modified one way
or another. Thanks.

Brian

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
> 
> _______________________________________________
> xfs mailing list
> xfs@oss.sgi.com
> http://oss.sgi.com/mailman/listinfo/xfs

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Re: [PATCH] xfs: optimise away log forces on timestamp updates for fdatasync

[Dave Chinner]

On Mon, Oct 26, 2015 at 07:49:29AM -0400, Brian Foster wrote:
> On Mon, Oct 26, 2015 at 04:07:20PM +1100, Dave Chinner wrote:
> > > 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)?
> > 
> > The flush is needed - fdatasync needs to guarantee the data is
> > on stable storage even if no metadata needs to be written to the
> > journal.
> > 
> 
> Ok. Well it's too bad we don't get any feedback about what was written
> from the filemap_write_and_wait_range() call. As it is, we send a flush
> even if there's nothing to write back.

Direct IO still needs cache flushes.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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