dm mpath: support discard

Enable discard support in the DM multipath target.

This discard support depends on a few discard-specific fixes to the
block layer's request stacking driver methods.

Discard requests are optional so don't allow a failed discard to trigger
path failures.  If there is a real problem with a given path the
barriers associated with the discard (either before or after the
discard) will cause path failure.  That said, unconditionally passing
discard failures up the stack is not ideal.  This must be fixed once DM
has more information about the nature of the underlying storage failure.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index da2223aa727f86da02c13629e9f559163f858b9b..487ecda90ad48f769643d03cf862cdff5ee8c76e 100644 (file)
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -893,6 +893,7 @@ static int multipath_ctr(struct dm_target *ti, unsigned int argc,
        }
 
        ti->num_flush_requests = 1;
+       ti->num_discard_requests = 1;
 
        return 0;
 
@@ -1272,6 +1273,15 @@ static int do_end_io(struct multipath *m, struct request *clone,
        if (error == -EOPNOTSUPP)
                return error;
 
+       if (clone->cmd_flags & REQ_DISCARD)
+               /*
+                * Pass all discard request failures up.
+                * FIXME: only fail_path if the discard failed due to a
+                * transport problem.  This requires precise understanding
+                * of the underlying failure (e.g. the SCSI sense).
+                */
+               return error;
+
        if (mpio->pgpath)
                fail_path(mpio->pgpath);