[PATCH 2/2] xfs: don't truncate prealloc from frequently accessed inodes

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

A long standing problem for streaming writeѕ through the NFS server
has been that the NFS server opens and closes file descriptors on an
inode for every write. The result of this behaviour is that the
->release() function is called on every close and that results in
XFS truncating speculative preallocation beyond the EOF.  This has
an adverse effect on file layout when multiple files are being
written at the same time - they interleave their extents and can
result in severe fragmentation.

To avoid this problem, keep a count of the number of ->release calls
made on an inode. For most cases, an inode is only going to be opened
once for writing and then closed again during it's lifetime in
cache. Hence if there are multiple ->release calls, there is a good
chance that the inode is being accessed by the NFS server. Hence
count up every time ->release is called while there are delalloc
blocks still outstanding on the inode.

If this count is non-zero when ->release is next called, then do no
truncate away the speculative preallocation - leave it there so that
subsequent writes do not need to reallocate the delalloc space. This
will prevent interleaving of extents of different inodes written
concurrently to the same AG.

If we get this wrong, it is not a big deal as we truncate
speculative allocation beyond EOF anyway in xfs_inactive() when the
inode is thrown out of the cache.

The new counter in the struct xfs_inode fits into a hole in the
structure on 64 bit machines, so does not grow the size of the inode
at all.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/xfs_inode.h    |    1 +
 fs/xfs/xfs_vnodeops.c |   15 ++++++++++++++-
 2 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
index 1594190..82aad5e 100644
--- a/fs/xfs/xfs_inode.h
+++ b/fs/xfs/xfs_inode.h
@@ -261,6 +261,7 @@ typedef struct xfs_inode {
 	xfs_fsize_t		i_size;		/* in-memory size */
 	xfs_fsize_t		i_new_size;	/* size when write completes */
 	atomic_t		i_iocount;	/* outstanding I/O count */
+	int			i_dirty_releases; /* dirty ->release calls */
 
 	/* VFS inode */
 	struct inode		i_vnode;	/* embedded VFS inode */
diff --git a/fs/xfs/xfs_vnodeops.c b/fs/xfs/xfs_vnodeops.c
index b7bdc43..0c8eeba 100644
--- a/fs/xfs/xfs_vnodeops.c
+++ b/fs/xfs/xfs_vnodeops.c
@@ -979,14 +979,27 @@ xfs_release(
 			 * chance to drop them once the last reference to
 			 * the inode is dropped, so we'll never leak blocks
 			 * permanently.
+			 *
+			 * Further, count the number of times we get here in
+			 * the life of this inode. If the inode is being
+			 * opened, written and closed frequently and we have
+			 * delayed allocation blocks oustanding (e.g. streaming
+			 * writes from the NFS server), truncating the
+			 * blocks past EOF will cause fragmentation to occur.
+			 * In this case don't do the truncation, either.
 			 */
+			if (ip->i_delayed_blks)
+				ip->i_dirty_releases++;
+			if (ip->i_dirty_releases > 1)
+					goto out;
+
 			error = xfs_free_eofblocks(mp, ip,
 						   XFS_FREE_EOF_TRYLOCK);
 			if (error)
 				return error;
 		}
 	}
-
+out:
 	return 0;
 }
 
-- 
1.7.1

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