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

MFC after:	1 week

2 files changed, 6 insertions, 4 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zfs_ioctl.h b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zfs_ioctl.h
index 5692dc6..9e9ac34 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zfs_ioctl.h
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zfs_ioctl.h
@@ -300,7 +300,8 @@ typedef struct zfs_cmd {
 	boolean_t	zc_temphold;
 	uint64_t	zc_action_handle;
 	int		zc_cleanup_fd;
-	uint8_t		zc_pad[4];		/* alignment */
+	uint8_t		zc_simple;
+	uint8_t		zc_pad[3];		/* alignment */
 	uint64_t	zc_sendobj;
 	uint64_t	zc_fromobj;
 	uint64_t	zc_createtxg;
diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c
index fa09170..15dcfde 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_ioctl.c
@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2011 Pawel Jakub Dawidek <pawel@dawidek.net>.
+ * Copyright (c) 2011-2012 Pawel Jakub Dawidek <pawel@dawidek.net>.
  * All rights reserved.
  * Portions Copyright 2011 Martin Matuska <mm@FreeBSD.org>
  * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
@@ -2036,6 +2036,7 @@ top:
  * zc_name		name of filesystem
  * zc_cookie		zap cursor
  * zc_nvlist_dst_size	size of buffer for property nvlist
+ * zc_simple		when set, only name is requested
  *
  * outputs:
  * zc_name		name of next snapshot
@@ -2050,7 +2051,7 @@ zfs_ioc_snapshot_list_next(zfs_cmd_t *zc)
 	int error;
 
 top:
-	if (snapshot_list_prefetch && zc->zc_cookie == 0)
+	if (snapshot_list_prefetch && zc->zc_cookie == 0 && !zc->zc_simple)
 		(void) dmu_objset_find(zc->zc_name, dmu_objset_prefetch,
 		    NULL, DS_FIND_SNAPSHOTS);
 
@@ -2072,7 +2073,7 @@ top:
 	    zc->zc_name + strlen(zc->zc_name), &zc->zc_obj, &zc->zc_cookie,
 	    NULL);
 
-	if (error == 0) {
+	if (error == 0 && !zc->zc_simple) {
 		dsl_dataset_t *ds;
 		dsl_pool_t *dp = os->os_dsl_dataset->ds_dir->dd_pool;