Avoid memory copies during mirror scrub.

Issuing several scrub reads for a block we may use the parent ZIO
buffer for one of child ZIOs.  If that read complete successfully,
then we won't need to copy the data explicitly.  If block has only
one copy (typical for root vdev, which is also a mirror inside),
then we never need to copy -- succeed or fail as-is.  Previous
code also copied data from buffer of every successfully completed
child ZIO, but that just does not make any sense.

On healthy N-wide mirror this saves all N+1 (or even more in case
of ditto blocks) memory copies for each scrubbed block, allowing
CPU to focus mostly on check-summing.  For other vdev types it
should save one memory copy per block copy at root vdev.

Signed-off-by: Alexander Motin <[email protected]>
Sponsored-By: iXsystems, Inc.

module/zfs/vdev_mirror.c

@@ -436,36 +436,6 @@ vdev_mirror_child_done(zio_t *zio)
 {
 	mirror_child_t *mc = zio->io_private;
 
-	/* See scrubbing read comment in vdev_mirror_io_start() */
-	if (zio->io_flags & ZIO_FLAG_SCRUB && zio->io_bp == NULL)
-		mc->mc_abd = zio->io_abd;
-
-	mc->mc_error = zio->io_error;
-	mc->mc_tried = 1;
-	mc->mc_skipped = 0;
-}
-
-static void
-vdev_mirror_scrub_done(zio_t *zio)
-{
-	mirror_child_t *mc = zio->io_private;
-
-	if (zio->io_error == 0) {
-		zio_t *pio;
-		zio_link_t *zl = NULL;
-
-		mutex_enter(&zio->io_lock);
-		while ((pio = zio_walk_parents(zio, &zl)) != NULL) {
-			mutex_enter(&pio->io_lock);
-			ASSERT3U(zio->io_size, >=, pio->io_size);
-			abd_copy(pio->io_abd, zio->io_abd, pio->io_size);
-			mutex_exit(&pio->io_lock);
-		}
-		mutex_exit(&zio->io_lock);
-	}
-
-	abd_free(zio->io_abd);
-
 	mc->mc_error = zio->io_error;
 	mc->mc_tried = 1;
 	mc->mc_skipped = 0;
@@ -645,15 +615,13 @@ vdev_mirror_io_start(zio_t *zio)
 	if (zio->io_type == ZIO_TYPE_READ) {
 		if ((zio->io_flags & ZIO_FLAG_SCRUB) && !mm->mm_resilvering) {
 			/*
-			 * For scrubbing reads we need to allocate a buffer
-			 * for each child and issue reads to all children.
-			 * If we can verify the checksum here, as indicated
-			 * by io_bp being non-NULL, the data will be copied
-			 * into zio->io_data in vdev_mirror_scrub_done().
-			 * If not, then vdev_mirror_io_done() will compare
-			 * all of the read buffers and return a checksum
-			 * error if they aren't all identical.
+			 * For scrubbing reads we need to issue reads to all
+			 * children.  One child can reuse parent buffer, but
+			 * for others we have to allocate separate ones to
+			 * verify checksums if io_bp is non-NULL, or compare
+			 * them in vdev_mirror_io_done() otherwise.
 			 */
+			boolean_t first = B_TRUE;
 			for (c = 0; c < mm->mm_children; c++) {
 				mc = &mm->mm_child[c];
 
@@ -665,13 +633,15 @@ vdev_mirror_io_start(zio_t *zio)
 					continue;
 				}
 
-				zio_nowait(zio_vdev_child_io(zio, zio->io_bp,
-				    mc->mc_vd, mc->mc_offset,
+				mc->mc_abd = first ? zio->io_abd :
 				    abd_alloc_sametype(zio->io_abd,
-				    zio->io_size), zio->io_size,
-				    zio->io_type, zio->io_priority, 0,
-				    zio->io_bp ? vdev_mirror_scrub_done :
+				    zio->io_size);
+				zio_nowait(zio_vdev_child_io(zio, zio->io_bp,
+				    mc->mc_vd, mc->mc_offset, mc->mc_abd,
+				    zio->io_size, zio->io_type,
+				    zio->io_priority, 0,
 				    vdev_mirror_child_done, mc));
+				first = B_FALSE;
 			}
 			zio_execute(zio);
 			return;
@@ -802,55 +772,76 @@ vdev_mirror_io_done(zio_t *zio)
 		return;
 	}
 
-	/*
-	 * If we're scrubbing but don't have a BP available (because this
-	 * vdev is under a raidz or draid vdev) then the best we can do is
-	 * compare all of the copies read.  If they're not identical then
-	 * return a checksum error and the most likely correct data.  The
-	 * raidz code will issue a repair I/O if possible.
-	 */
-	if (zio->io_flags & ZIO_FLAG_SCRUB && zio->io_bp == NULL) {
-		abd_t *last_good_abd;
-
-		ASSERT(zio->io_vd->vdev_ops == &vdev_replacing_ops ||
-		    zio->io_vd->vdev_ops == &vdev_spare_ops);
+	if (zio->io_flags & ZIO_FLAG_SCRUB && !mm->mm_resilvering) {
+		abd_t *best_abd = NULL;
+		if (last_good_copy >= 0)
+			best_abd = mm->mm_child[last_good_copy].mc_abd;
 
-		if (good_copies > 1) {
-			last_good_abd = mm->mm_child[last_good_copy].mc_abd;
-			abd_t *best_abd = NULL;
+		/*
+		 * If we're scrubbing but don't have a BP available (because
+		 * this vdev is under a raidz or draid vdev) then the best we
+		 * can do is compare all of the copies read.  If they're not
+		 * identical then return a checksum error and the most likely
+		 * correct data.  The raidz code will issue a repair I/O if
+		 * possible.
+		 */
+		if (zio->io_bp == NULL) {
+			ASSERT(zio->io_vd->vdev_ops == &vdev_replacing_ops ||
+			    zio->io_vd->vdev_ops == &vdev_spare_ops);
 
+			abd_t *pref_abd = NULL;
 			for (c = 0; c < last_good_copy; c++) {
 				mc = &mm->mm_child[c];
-
 				if (mc->mc_error || !mc->mc_tried)
 					continue;
 
-				if (abd_cmp(mc->mc_abd, last_good_abd) != 0)
+				if (abd_cmp(mc->mc_abd, best_abd) != 0)
 					zio->io_error = SET_ERROR(ECKSUM);
 
 				/*
 				 * The distributed spare is always prefered
 				 * by vdev_mirror_child_select() so it's
 				 * considered to be the best candidate.
 				 */
-				if (best_abd == NULL &&
+				if (pref_abd == NULL &&
 				    mc->mc_vd->vdev_ops ==
-				    &vdev_draid_spare_ops) {
+				    &vdev_draid_spare_ops)
+					pref_abd = mc->mc_abd;
+
+				/*
+				 * In the absence of a preferred copy, use
+				 * the parent pointer to avoid a memory copy.
+				 */
+				if (mc->mc_abd == zio->io_abd)
 					best_abd = mc->mc_abd;
-				}
 			}
+			if (pref_abd)
+				best_abd = pref_abd;
+		} else {
 
-			abd_copy(zio->io_abd, best_abd ? best_abd :
-			    last_good_abd, zio->io_size);
-
-		} else if (good_copies == 1) {
-			last_good_abd = mm->mm_child[last_good_copy].mc_abd;
-			abd_copy(zio->io_abd, last_good_abd, zio->io_size);
+			/*
+			 * If we have a BP available, then checksums are
+			 * already verified and we just need a buffer
+			 * with valid data, preferring parent one to
+			 * avoid a memory copy.
+			 */
+			for (c = 0; c < last_good_copy; c++) {
+				mc = &mm->mm_child[c];
+				if (mc->mc_error || !mc->mc_tried)
+					continue;
+				if (mc->mc_abd == zio->io_abd) {
+					best_abd = mc->mc_abd;
+					break;
+				}
+			}
 		}
 
+		if (best_abd && best_abd != zio->io_abd)
+			abd_copy(zio->io_abd, best_abd, zio->io_size);
 		for (c = 0; c < mm->mm_children; c++) {
 			mc = &mm->mm_child[c];
-			abd_free(mc->mc_abd);
+			if (mc->mc_abd != zio->io_abd)
+				abd_free(mc->mc_abd);
 			mc->mc_abd = NULL;
 		}
 	}