xfs: don't discard on free of unwritten extents

Source kernel commit: 84ca484ecf2f8e1dc3afddc895cb9b62c531db49

Unwritten extents by definition have not been written to until they
are converted to normal written extents. If unwritten extents are
freed from a file, it is therefore guaranteed that the blocks have
not been written to since allocation (note that zero range punches
and reallocates blocks).

To cut down on online discards generated from workloads that make
use of preallocation, skip discards of extents if they are in the
unwritten state when the extent is freed.

Note that this optimization does not apply to log recovery, during
which all freed extents are discarded if online discard is enabled.
Also note that it may be possible for a filesystem crash to occur
after write completion of an unwritten extent but before unwritten
conversion such that the extent remains unwritten after log
recovery. Since this pseudo-inconsistency may already be possible
after a crash (consider writing to recently allocated blocks where
the allocation transaction is lost after a crash), this change
shouldn't introduce any fundamental limitations that don't already
exist. In short, on storage stacks where discards are important,
it's good practice to run an occasional fstrim even with online
discard enabled in the filesystem, particularly after a crash.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>

diff --git a/libxfs/xfs_bmap.c b/libxfs/xfs_bmap.c
index d8f54deaf27e82cc23673a254e0c64f8caf5466a..f894aff909a62779d76b063ff9a63799728aafe4 100644 (file)
--- a/libxfs/xfs_bmap.c
+++ b/libxfs/xfs_bmap.c
@@ -5100,7 +5100,8 @@ xfs_bmap_del_extent_real(
                        if (error)
                                goto done;
                } else {
-                       if (bflags & XFS_BMAPI_NODISCARD) {
+                       if ((bflags & XFS_BMAPI_NODISCARD) ||
+                           (del->br_state == XFS_EXT_UNWRITTEN)) {
                                xfs_bmap_add_free_nodiscard(mp, dfops,
                                        del->br_startblock, del->br_blockcount,
                                        NULL);