Parallelize vdev_load

metaslab_init is the slowest part of importing a mature pool, and it
must be repeated hundreds of times for each top-level vdev.  But its
speed is dominated by a few serialized disk accesses.  That can lead to
import times of > 1 hour for pools with many top-level vdevs on spinny
disks.

Speed up the import by using a taskqueue to parallelize vdev_load across
all top-level vdevs.

This also requires adding mutex protection to
metaslab_class_t.mc_historgram.  The mc_histogram fields were
unprotected when that code was first written in "Illumos 4976-4984 -
metaslab improvements" (OpenZFS
f3a7f6610f2df0217ba3b99099019417a954b673).  The lock wasn't added until
3dfb57a35e8cbaa7c424611235d669f3c575ada1, though it's unclear exactly
which fields it's supposed to protect.  In any case, it wasn't until
vdev_load was parallelized that any code attempted concurrent access to
those fields.

Sponsored by: Axcient
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Alan Somers <[email protected]>
Closes #11470

1 files changed, 39 insertions, 3 deletions

diff --git a/module/zfs/vdev.c b/module/zfs/vdev.c
index f305da6f5..018e48c38 100644
--- a/module/zfs/vdev.c
+++ b/module/zfs/vdev.c
@@ -1725,6 +1725,14 @@ vdev_probe(vdev_t *vd, zio_t *zio)
 }
 
 static void
+vdev_load_child(void *arg)
+{
+	vdev_t *vd = arg;
+
+	vd->vdev_load_error = vdev_load(vd);
+}
+
+static void
 vdev_open_child(void *arg)
 {
 	vdev_t *vd = arg;
@@ -3350,18 +3358,46 @@ vdev_checkpoint_sm_object(vdev_t *vd, uint64_t *sm_obj)
 int
 vdev_load(vdev_t *vd)
 {
+	int children = vd->vdev_children;
 	int error = 0;
+	taskq_t *tq = NULL;
+
+	/*
+	 * It's only worthwhile to use the taskq for the root vdev, because the
+	 * slow part is metaslab_init, and that only happens for top-level
+	 * vdevs.
+	 */
+	if (vd->vdev_ops == &vdev_root_ops && vd->vdev_children > 0) {
+		tq = taskq_create("vdev_load", children, minclsyspri,
+		    children, children, TASKQ_PREPOPULATE);
+	}
 
 	/*
 	 * Recursively load all children.
 	 */
 	for (int c = 0; c < vd->vdev_children; c++) {
-		error = vdev_load(vd->vdev_child[c]);
-		if (error != 0) {
-			return (error);
+		vdev_t *cvd = vd->vdev_child[c];
+
+		if (tq == NULL || vdev_uses_zvols(cvd)) {
+			cvd->vdev_load_error = vdev_load(cvd);
+		} else {
+			VERIFY(taskq_dispatch(tq, vdev_load_child,
+			    cvd, TQ_SLEEP) != TASKQID_INVALID);
 		}
 	}
 
+	if (tq != NULL) {
+		taskq_wait(tq);
+		taskq_destroy(tq);
+	}
+
+	for (int c = 0; c < vd->vdev_children; c++) {
+		int error = vd->vdev_child[c]->vdev_load_error;
+
+		if (error != 0)
+			return (error);
+	}
+
 	vdev_set_deflate_ratio(vd);
 
 	/*