Add createtxg sort support for simple snapshot iterator

- When iterating snapshots with name only, e.g., "-o name -s name",
libzfs uses simple snapshot iterator and results are displayed
in alphabetic order. This PR adds support for faster version of
createtxg sort by avoiding nvlist parsing for properties. Flags
"-o name -s createtxg" will enable createtxg sort while using
simple snapshot iterator.
- Added support to read createtxg property directly from zfs handle
for filesystem, volume and snapshot types instead of parsing nvlist.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #13577

cmd/zfs/zfs_iter.c

@@ -218,6 +218,13 @@ zfs_sort_only_by_name(const zfs_sort_column_t *sc)
 	    sc->sc_prop == ZFS_PROP_NAME);
 }
 
+int
+zfs_sort_only_by_createtxg(const zfs_sort_column_t *sc)
+{
+	return (sc != NULL && sc->sc_next == NULL &&
+	    sc->sc_prop == ZFS_PROP_CREATETXG);
+}
+
 static int
 zfs_compare(const void *larg, const void *rarg)
 {
@@ -301,7 +308,7 @@ zfs_sort(const void *larg, const void *rarg, void *data)
 	for (psc = sc; psc != NULL; psc = psc->sc_next) {
 		char lbuf[ZFS_MAXPROPLEN], rbuf[ZFS_MAXPROPLEN];
 		char *lstr, *rstr;
-		uint64_t lnum, rnum;
+		uint64_t lnum = 0, rnum = 0;
 		boolean_t lvalid, rvalid;
 		int ret = 0;
 
@@ -352,11 +359,9 @@ zfs_sort(const void *larg, const void *rarg, void *data)
 			    zfs_get_type(r), B_FALSE);
 
 			if (lvalid)
-				(void) zfs_prop_get_numeric(l, psc->sc_prop,
+				lnum = zfs_prop_get_int(l, psc->sc_prop);
-				    &lnum, NULL, NULL, 0);
 			if (rvalid)
-				(void) zfs_prop_get_numeric(r, psc->sc_prop,
+				rnum = zfs_prop_get_int(r, psc->sc_prop);
-				    &rnum, NULL, NULL, 0);
 		}
 
 		if (!lvalid && !rvalid)

cmd/zfs/zfs_iter.h

@@ -53,6 +53,7 @@ int zfs_for_each(int, char **, int options, zfs_type_t,
 int zfs_add_sort_column(zfs_sort_column_t **, const char *, boolean_t);
 void zfs_free_sort_columns(zfs_sort_column_t *);
 int zfs_sort_only_by_name(const zfs_sort_column_t *);
+int zfs_sort_only_by_createtxg(const zfs_sort_column_t *);
 
 #ifdef	__cplusplus
 }

cmd/zfs/zfs_main.c

@@ -3654,11 +3654,14 @@ found3:;
 	argv += optind;
 
 	/*
-	 * If we are only going to list snapshot names and sort by name,
+	 * If we are only going to list snapshot names and sort by name or
-	 * then we can use faster version.
+	 * by createtxg, then we can use faster version.
 	 */
-	if (strcmp(fields, "name") == 0 && zfs_sort_only_by_name(sortcol))
+	if (strcmp(fields, "name") == 0 &&
+	    (zfs_sort_only_by_name(sortcol) ||
+	    zfs_sort_only_by_createtxg(sortcol))) {
 		flags |= ZFS_ITER_SIMPLE;
+	}
 
 	/*
 	 * If "-o space" and no types were specified, don't display snapshots.

lib/libzfs/libzfs_dataset.c

@@ -527,6 +527,7 @@ make_dataset_simple_handle_zc(zfs_handle_t *pzhp, zfs_cmd_t *zc)
 	zhp->zfs_head_type = pzhp->zfs_type;
 	zhp->zfs_type = ZFS_TYPE_SNAPSHOT;
 	zhp->zpool_hdl = zpool_handle(zhp);
+	zhp->zfs_dmustats = zc->zc_objset_stats;
 
 	return (zhp);
 }
@@ -2283,6 +2284,19 @@ get_numeric_property(zfs_handle_t *zhp, zfs_prop_t prop, zprop_source_t *src,
 		*val = zhp->zfs_dmustats.dds_redacted;
 		break;
 
+	case ZFS_PROP_CREATETXG:
+		/*
+		 * We can directly read createtxg property from zfs
+		 * handle for Filesystem, Snapshot and ZVOL types.
+		 */
+		if ((zhp->zfs_type == ZFS_TYPE_FILESYSTEM) ||
+		    (zhp->zfs_type == ZFS_TYPE_SNAPSHOT) ||
+		    (zhp->zfs_type == ZFS_TYPE_VOLUME)) {
+			*val = zhp->zfs_dmustats.dds_creation_txg;
+			break;
+		}
+		zfs_fallthrough;
+
 	default:
 		switch (zfs_prop_get_type(prop)) {
 		case PROP_TYPE_NUMBER:

module/zfs/zfs_ioctl.c

@@ -2322,6 +2322,8 @@ zfs_ioc_snapshot_list_next(zfs_cmd_t *zc)
 		}
 
 		if (zc->zc_simple) {
+			zc->zc_objset_stats.dds_creation_txg =
+			    dsl_get_creationtxg(ds);
 			dsl_dataset_rele(ds, FTAG);
 			break;
 		}