Report ashift of L2ARC devices in zdb

Commit 8af1104 does not actually store the ashift of cache devices in
their label. However, in order to facilitate reporting the ashift
through zdb, we enable this in the present commit. We also document
how the retrieval of the ashift is done.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #15331

cmd/zdb/zdb.c

@@ -4815,7 +4815,7 @@ dump_label(const char *dev)
 			if (nvlist_size(config, &size, NV_ENCODE_XDR) != 0)
 				size = buflen;
 
-			/* If the device is a cache device clear the header. */
+			/* If the device is a cache device read the header. */
 			if (!read_l2arc_header) {
 				if (nvlist_lookup_uint64(config,
 				    ZPOOL_CONFIG_POOL_STATE, &l2cache) == 0 &&

module/zfs/vdev_label.c

@@ -1103,6 +1103,16 @@ vdev_label_init(vdev_t *vd, uint64_t crtxg, vdev_labeltype_t reason)
 		    POOL_STATE_L2CACHE) == 0);
 		VERIFY(nvlist_add_uint64(label, ZPOOL_CONFIG_GUID,
 		    vd->vdev_guid) == 0);
+
+		/*
+		 * This is merely to facilitate reporting the ashift of the
+		 * cache device through zdb. The actual retrieval of the
+		 * ashift (in vdev_alloc()) uses the nvlist
+		 * spa->spa_l2cache->sav_config (populated in
+		 * spa_ld_open_aux_vdevs()).
+		 */
+		VERIFY(nvlist_add_uint64(label, ZPOOL_CONFIG_ASHIFT,
+		    vd->vdev_ashift) == 0);
 	} else {
 		uint64_t txg = 0ULL;