OpenZFS 9690 - metaslab of vdev with no space maps was flushed during…

… removal

Authored by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/9690
OpenZFS-commit: openzfs/openzfs@4e75ba6826
Closes #8039

module/zfs/vdev.c

@@ -3129,11 +3129,11 @@ vdev_destroy_spacemaps(vdev_t *vd, dmu_tx_t *tx)
 }
 
 static void
-vdev_remove_empty(vdev_t *vd, uint64_t txg)
+vdev_remove_empty_log(vdev_t *vd, uint64_t txg)
 {
 	spa_t *spa = vd->vdev_spa;
-	dmu_tx_t *tx;
 
+	ASSERT(vd->vdev_islog);
 	ASSERT(vd == vd->vdev_top);
 	ASSERT3U(txg, ==, spa_syncing_txg(spa));
 
@@ -3178,13 +3178,14 @@ vdev_remove_empty(vdev_t *vd, uint64_t txg)
 			ASSERT0(mg->mg_histogram[i]);
 	}
 
-	tx = dmu_tx_create_assigned(spa_get_dsl(spa), txg);
-	vdev_destroy_spacemaps(vd, tx);
+	dmu_tx_t *tx = dmu_tx_create_assigned(spa_get_dsl(spa), txg);
 
-	if (vd->vdev_islog && vd->vdev_top_zap != 0) {
+	vdev_destroy_spacemaps(vd, tx);
+	if (vd->vdev_top_zap != 0) {
 		vdev_destroy_unlink_zap(vd, vd->vdev_top_zap, tx);
 		vd->vdev_top_zap = 0;
 	}
+
 	dmu_tx_commit(tx);
 }
 
@@ -3255,14 +3256,11 @@ vdev_sync(vdev_t *vd, uint64_t txg)
 		vdev_dtl_sync(lvd, txg);
 
 	/*
-	 * Remove the metadata associated with this vdev once it's empty.
-	 * Note that this is typically used for log/cache device removal;
-	 * we don't empty toplevel vdevs when removing them.  But if
-	 * a toplevel happens to be emptied, this is not harmful.
+	 * If this is an empty log device being removed, destroy the
+	 * metadata associated with it.
 	 */
-	if (vd->vdev_stat.vs_alloc == 0 && vd->vdev_removing) {
-		vdev_remove_empty(vd, txg);
-	}
+	if (vd->vdev_islog && vd->vdev_stat.vs_alloc == 0 && vd->vdev_removing)
+		vdev_remove_empty_log(vd, txg);
 
 	(void) txg_list_add(&spa->spa_vdev_txg_list, vd, TXG_CLEAN(txg));
 }