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 <[email protected]>
Issue #450

1 files changed, 3 insertions, 2 deletions

diff --git a/module/zfs/zfs_ioctl.c b/module/zfs/zfs_ioctl.c
index 17dc35d74..a232ed0d5 100644
--- a/module/zfs/zfs_ioctl.c
+++ b/module/zfs/zfs_ioctl.c
@@ -21,6 +21,7 @@
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
  * Portions Copyright 2011 Martin Matuska
+ * Portions Copyright 2012 Pawel Jakub Dawidek <[email protected]>
  * Copyright (c) 2012, Joyent, Inc. All rights reserved.
  */
 
@@ -1973,7 +1974,7 @@ zfs_ioc_snapshot_list_next(zfs_cmd_t *zc)
 	int error;
 
 top:
-	if (zc->zc_cookie == 0)
+	if (zc->zc_cookie == 0 && !zc->zc_simple)
 		(void) dmu_objset_find(zc->zc_name, dmu_objset_prefetch,
 		    NULL, DS_FIND_SNAPSHOTS);
 
@@ -1995,7 +1996,7 @@ 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;