fix: Uber block label not always found for aux vdevs

When spare or l2cache (aux) vdev is added during pool creation,
spa->spa_uberblock is not dumped until that point. Subsequently,
the aux label is never synchronized after its initial creation,
resulting in the uberblock label remaining undumped. The uberblock
is crucial for lib_blkid in identifying the ZFS partition type. To
address this issue, we now ensure sync of the uberblock label once
if it's not dumped initially.

Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #15737

include/sys/spa_impl.h

@@ -278,6 +278,7 @@ struct spa {
 
 	spa_aux_vdev_t	spa_spares;		/* hot spares */
 	spa_aux_vdev_t	spa_l2cache;		/* L2ARC cache devices */
+	boolean_t	spa_aux_sync_uber;	/* need to sync aux uber */
 	nvlist_t	*spa_label_features;	/* Features for reading MOS */
 	uint64_t	spa_config_object;	/* MOS object for pool config */
 	uint64_t	spa_config_generation;	/* config generation number */

module/zfs/vdev_label.c

@@ -1156,6 +1156,14 @@ vdev_label_init(vdev_t *vd, uint64_t crtxg, vdev_labeltype_t reason)
 		 */
 		VERIFY(nvlist_add_uint64(label, ZPOOL_CONFIG_ASHIFT,
 		    vd->vdev_ashift) == 0);
+
+		/*
+		 * When spare or l2cache (aux) vdev is added during pool
+		 * creation, spa->spa_uberblock is not written until this
+		 * point. Write it on next config sync.
+		 */
+		if (uberblock_verify(&spa->spa_uberblock))
+			spa->spa_aux_sync_uber = B_TRUE;
 	} else {
 		uint64_t txg = 0ULL;
 
@@ -1794,6 +1802,16 @@ vdev_uberblock_sync_list(vdev_t **svd, int svdcount, uberblock_t *ub, int flags)
 	for (int v = 0; v < svdcount; v++)
 		vdev_uberblock_sync(zio, &good_writes, ub, svd[v], flags);
 
+	if (spa->spa_aux_sync_uber) {
+		for (int v = 0; v < spa->spa_spares.sav_count; v++) {
+			vdev_uberblock_sync(zio, &good_writes, ub,
+			    spa->spa_spares.sav_vdevs[v], flags);
+		}
+		for (int v = 0; v < spa->spa_l2cache.sav_count; v++) {
+			vdev_uberblock_sync(zio, &good_writes, ub,
+			    spa->spa_l2cache.sav_vdevs[v], flags);
+		}
+	}
 	(void) zio_wait(zio);
 
 	/*
@@ -1808,6 +1826,19 @@ vdev_uberblock_sync_list(vdev_t **svd, int svdcount, uberblock_t *ub, int flags)
 			zio_flush(zio, svd[v]);
 		}
 	}
+	if (spa->spa_aux_sync_uber) {
+		spa->spa_aux_sync_uber = B_FALSE;
+		for (int v = 0; v < spa->spa_spares.sav_count; v++) {
+			if (vdev_writeable(spa->spa_spares.sav_vdevs[v])) {
+				zio_flush(zio, spa->spa_spares.sav_vdevs[v]);
+			}
+		}
+		for (int v = 0; v < spa->spa_l2cache.sav_count; v++) {
+			if (vdev_writeable(spa->spa_l2cache.sav_vdevs[v])) {
+				zio_flush(zio, spa->spa_l2cache.sav_vdevs[v]);
+			}
+		}
+	}
 
 	(void) zio_wait(zio);