1 files changed, 26 insertions, 4 deletions

diff --git a/drivers/block/drbd/drbd_req.c b/drivers/block/drbd/drbd_req.c
index 910335c30927..0bb1e41f136f 100644
--- a/drivers/block/drbd/drbd_req.c
+++ b/drivers/block/drbd/drbd_req.c
@@ -834,7 +834,15 @@ static int drbd_make_request_common(struct drbd_conf *mdev, struct bio *bio, uns
                 req->private_bio = NULL;
         }
         if (rw == WRITE) {
-                remote = 1;
+                /* Need to replicate writes.  Unless it is an empty flush,
+                 * which is better mapped to a DRBD P_BARRIER packet,
+                 * also for drbd wire protocol compatibility reasons. */
+                if (unlikely(size == 0)) {
+                        /* The only size==0 bios we expect are empty flushes. */
+                        D_ASSERT(bio->bi_rw & REQ_FLUSH);
+                        remote = 0;
+                } else
+                        remote = 1;
         } else {
                 /* READ || READA */
                 if (local) {
@@ -870,8 +878,11 @@ static int drbd_make_request_common(struct drbd_conf *mdev, struct bio *bio, uns
          * extent.  This waits for any resync activity in the corresponding
          * resync extent to finish, and, if necessary, pulls in the target
          * extent into the activity log, which involves further disk io because
-         * of transactional on-disk meta data updates. */
-        if (rw == WRITE && local && !test_bit(AL_SUSPENDED, &mdev->flags)) {
+         * of transactional on-disk meta data updates.
+         * Empty flushes don't need to go into the activity log, they can only
+         * flush data for pending writes which are already in there. */
+        if (rw == WRITE && local && size
+        && !test_bit(AL_SUSPENDED, &mdev->flags)) {
                 req->rq_state |= RQ_IN_ACT_LOG;
                 drbd_al_begin_io(mdev, sector);
         }
@@ -994,7 +1005,10 @@ allocate_barrier:
         if (rw == WRITE && _req_conflicts(req))
                 goto fail_conflicting;
 
-        list_add_tail(&req->tl_requests, &mdev->newest_tle->requests);
+        /* no point in adding empty flushes to the transfer log,
+         * they are mapped to drbd barriers already. */
+        if (likely(size!=0))
+                list_add_tail(&req->tl_requests, &mdev->newest_tle->requests);
 
         /* NOTE remote first: to get the concurrent write detection right,
          * we must register the request before start of local IO.  */
@@ -1014,6 +1028,14 @@ allocate_barrier:
             mdev->net_conf->on_congestion != OC_BLOCK && mdev->agreed_pro_version >= 96)
                 maybe_pull_ahead(mdev);
 
+        /* If this was a flush, queue a drbd barrier/start a new epoch.
+         * Unless the current epoch was empty anyways, or we are not currently
+         * replicating, in which case there is no point. */
+        if (unlikely(bio->bi_rw & REQ_FLUSH)
+                && mdev->newest_tle->n_writes
+                && drbd_should_do_remote(mdev->state))
+                queue_barrier(mdev);
+
         spin_unlock_irq(&mdev->req_lock);
         kfree(b); /* if someone else has beaten us to it... */