Skip spacemaps reading in case of pool readonly import

The only zdb utility require to read metaslab-related data during
read-only pool import because of spacemaps validation. Add global
variable which will allow zdb read spacemaps in case of readonly
import mode.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #9095
Closes #12687

module/zfs/metaslab.c

@@ -2661,7 +2661,8 @@ metaslab_init(metaslab_group_t *mg, uint64_t id, uint64_t object,
 
 	/*
 	 * We only open space map objects that already exist. All others
-	 * will be opened when we finally allocate an object for it.
+	 * will be opened when we finally allocate an object for it. For
+	 * readonly pools there is no need to open the space map object.
 	 *
 	 * Note:
 	 * When called from vdev_expand(), we can't call into the DMU as
@@ -2670,7 +2671,8 @@ metaslab_init(metaslab_group_t *mg, uint64_t id, uint64_t object,
 	 * that case, the object parameter is zero though, so we won't
 	 * call into the DMU.
 	 */
-	if (object != 0) {
+	if (object != 0 && !(spa->spa_mode == SPA_MODE_READ &&
+	    !spa->spa_read_spacemaps)) {
 		error = space_map_open(&ms->ms_sm, mos, object, ms->ms_start,
 		    ms->ms_size, vd->vdev_ashift);
 

module/zfs/spa.c

@@ -180,6 +180,12 @@ boolean_t	spa_create_process = B_TRUE;	/* no process ==> no sysdc */
  */
 boolean_t	spa_load_verify_dryrun = B_FALSE;
 
+/*
+ * Allow read spacemaps in case of readonly import (spa_mode == SPA_MODE_READ).
+ * This is used by zdb for spacemaps verification.
+ */
+boolean_t	spa_mode_readable_spacemaps = B_FALSE;
+
 /*
  * This (illegal) pool name is used when temporarily importing a spa_t in order
  * to get the vdev stats associated with the imported devices.
@@ -1242,6 +1248,7 @@ spa_activate(spa_t *spa, spa_mode_t mode)
 
 	spa->spa_state = POOL_STATE_ACTIVE;
 	spa->spa_mode = mode;
+	spa->spa_read_spacemaps = spa_mode_readable_spacemaps;
 
 	spa->spa_normal_class = metaslab_class_create(spa, zfs_metaslab_ops);
 	spa->spa_log_class = metaslab_class_create(spa, zfs_metaslab_ops);

module/zfs/vdev.c

@@ -3142,6 +3142,12 @@ vdev_dtl_load(vdev_t *vd)
 	if (vd->vdev_ops->vdev_op_leaf && vd->vdev_dtl_object != 0) {
 		ASSERT(vdev_is_concrete(vd));
 
+		/*
+		 * If the dtl cannot be sync'd there is no need to open it.
+		 */
+		if (spa->spa_mode == SPA_MODE_READ && !spa->spa_read_spacemaps)
+			return (0);
+
 		error = space_map_open(&vd->vdev_dtl_sm, mos,
 		    vd->vdev_dtl_object, 0, -1ULL, 0);
 		if (error)