Speed up 'zfs list -t snapshot -o name -s name'

FreeBSD #xxx:  Dramatically optimize listing snapshots when user
requests only snapshot names and wants to sort them by name, ie.
when executes:

  # zfs list -t snapshot -o name -s name

Because only name is needed we don't have to read all snapshot
properties.

Below you can find how long does it take to list 34509 snapshots
from a single disk pool before and after this change with cold and
warm cache:

    before:

        # time zfs list -t snapshot -o name -s name > /dev/null
        cold cache: 525s
        warm cache: 218s

    after:

        # time zfs list -t snapshot -o name -s name > /dev/null
        cold cache: 1.7s
        warm cache: 1.1s

NOTE: This patch only appears in FreeBSD.  If/when Illumos picks up
the change we may want to drop this patch and adopt their version.
However, for now this addresses a real issue.

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #450

cmd/zfs/zfs_iter.c

@@ -20,6 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012 Pawel Jakub Dawidek <pawel@dawidek.net>.
  */
 
 #include <libintl.h>
@@ -129,8 +130,11 @@ zfs_callback(zfs_handle_t *zhp, void *data)
 		cb->cb_depth++;
 		if (zfs_get_type(zhp) == ZFS_TYPE_FILESYSTEM)
 			(void) zfs_iter_filesystems(zhp, zfs_callback, data);
-		if ((zfs_get_type(zhp) != ZFS_TYPE_SNAPSHOT) && include_snaps)
-			(void) zfs_iter_snapshots(zhp, zfs_callback, data);
+		if ((zfs_get_type(zhp) != ZFS_TYPE_SNAPSHOT) && include_snaps) {
+			(void) zfs_iter_snapshots(zhp,
+			    (cb->cb_flags & ZFS_ITER_SIMPLE) != 0, zfs_callback,
+			    data);
+		}
 		cb->cb_depth--;
 	}
 
@@ -184,6 +188,13 @@ zfs_free_sort_columns(zfs_sort_column_t *sc)
 	}
 }
 
+int
+zfs_sort_only_by_name(const zfs_sort_column_t *sc)
+{
+	return (sc != NULL && sc->sc_next == NULL &&
+	    sc->sc_prop == ZFS_PROP_NAME);
+}
+
 /* ARGSUSED */
 static int
 zfs_compare(const void *larg, const void *rarg, void *unused)
@@ -224,7 +235,13 @@ zfs_compare(const void *larg, const void *rarg, void *unused)
 			lcreate = zfs_prop_get_int(l, ZFS_PROP_CREATETXG);
 			rcreate = zfs_prop_get_int(r, ZFS_PROP_CREATETXG);
 
-			if (lcreate < rcreate)
+			/*
+			 * Both lcreate and rcreate being 0 means we don't have
+			 * properties and we should compare full name.
+			 */
+			if (lcreate == 0 && rcreate == 0)
+				ret = strcmp(lat + 1, rat + 1);
+			else if (lcreate < rcreate)
 				ret = -1;
 			else if (lcreate > rcreate)
 				ret = 1;
@@ -290,7 +307,14 @@ zfs_sort(const void *larg, const void *rarg, void *data)
 			if (rvalid)
 				verify(nvlist_lookup_string(rval,
 				    ZPROP_VALUE, &rstr) == 0);
+		} else if (psc->sc_prop == ZFS_PROP_NAME) {
+			lvalid = rvalid = B_TRUE;
 
+			(void) strlcpy(lbuf, zfs_get_name(l), sizeof(lbuf));
+			(void) strlcpy(rbuf, zfs_get_name(r), sizeof(rbuf));
+
+			lstr = lbuf;
+			rstr = rbuf;
 		} else if (zfs_prop_is_string(psc->sc_prop)) {
 			lvalid = (zfs_prop_get(l, psc->sc_prop, lbuf,
 			    sizeof (lbuf), NULL, NULL, 0, B_TRUE) == 0);

cmd/zfs/zfs_iter.h

@@ -43,11 +43,13 @@ typedef struct zfs_sort_column {
 #define	ZFS_ITER_PROP_LISTSNAPS    (1 << 2)
 #define	ZFS_ITER_DEPTH_LIMIT	   (1 << 3)
 #define	ZFS_ITER_RECVD_PROPS	   (1 << 4)
+#define	ZFS_ITER_SIMPLE		   (1 << 5)
 
 int zfs_for_each(int, char **, int options, zfs_type_t,
     zfs_sort_column_t *, zprop_list_t **, int, zfs_iter_f, void *);
 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 *);
 
 #ifdef	__cplusplus
 }

cmd/zfs/zfs_main.c

@@ -2650,7 +2650,12 @@ print_dataset(zfs_handle_t *zhp, zprop_list_t *pl, boolean_t scripted)
 			first = B_FALSE;
 		}
 
-		if (pl->pl_prop != ZPROP_INVAL) {
+		if (pl->pl_prop == ZFS_PROP_NAME) {
+			(void) strlcpy(property, zfs_get_name(zhp),
+			    sizeof(property));
+			propstr = property;
+			right_justify = zfs_prop_align_right(pl->pl_prop);
+		} else if (pl->pl_prop != ZPROP_INVAL) {
 			if (zfs_prop_get(zhp, pl->pl_prop, property,
 			    sizeof (property), NULL, NULL, 0, B_FALSE) != 0)
 				propstr = "-";
@@ -2810,6 +2815,13 @@ zfs_do_list(int argc, char **argv)
 	if (fields == NULL)
 		fields = default_fields;
 
+	/*
+	 * If we are only going to list snapshot names and sort by name,
+	 * then we can use faster version.
+	 */
+	if (strcmp(fields, "name") == 0 && zfs_sort_only_by_name(sortcol))
+		flags |= ZFS_ITER_SIMPLE;
+
 	/*
 	 * If "-o space" and no types were specified, don't display snapshots.
 	 */