Add snapshots_changed as property

Make dd_snap_cmtime property persistent across mount and unmount
operations by storing in ZAP and restore the value from ZAP on hold
into dd_snap_cmtime instead of updating it.

Expose dd_snap_cmtime as 'snapshots_changed' property that provides a
mechanism to quickly determine whether snapshot list for dataset has
changed without having to mount a dataset or iterate the snapshot list.

It specifies the time at which a snapshot for a dataset was last
created or deleted. This allows us to be more efficient how often we
query snapshots.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #13635

tests/runfiles/common.run

@@ -869,7 +869,7 @@ tests = ['clone_001_pos', 'rollback_001_pos', 'rollback_002_pos',
     'snapshot_006_pos', 'snapshot_007_pos', 'snapshot_008_pos',
     'snapshot_009_pos', 'snapshot_010_pos', 'snapshot_011_pos',
     'snapshot_012_pos', 'snapshot_013_pos', 'snapshot_014_pos',
-    'snapshot_017_pos']
+    'snapshot_017_pos', 'snapshot_018_pos']
 tags = ['functional', 'snapshot']
 
 [tests/functional/snapused]

tests/runfiles/sanity.run

@@ -568,7 +568,8 @@ tests = ['clone_001_pos', 'rollback_001_pos', 'rollback_002_pos',
     'snapshot_004_pos', 'snapshot_005_pos', 'snapshot_006_pos',
     'snapshot_007_pos', 'snapshot_008_pos', 'snapshot_009_pos',
     'snapshot_010_pos', 'snapshot_011_pos', 'snapshot_012_pos',
-    'snapshot_013_pos', 'snapshot_014_pos', 'snapshot_017_pos']
+    'snapshot_013_pos', 'snapshot_014_pos', 'snapshot_017_pos',
+    'snapshot_018_pos']
 tags = ['functional', 'snapshot']
 
 [tests/functional/snapused]

tests/zfs-tests/include/libtest.shlib

@@ -3432,6 +3432,20 @@ function stat_size #<path>
 	esac
 }
 
+function stat_mtime #<path>
+{
+	typeset path=$1
+
+	case "$UNAME" in
+	FreeBSD)
+		stat -f %m "$path"
+		;;
+	*)
+		stat -c %Y "$path"
+		;;
+	esac
+}
+
 function stat_ctime #<path>
 {
 	typeset path=$1

tests/zfs-tests/tests/Makefile.am

@@ -1857,6 +1857,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/snapshot/snapshot_015_pos.ksh \
 	functional/snapshot/snapshot_016_pos.ksh \
 	functional/snapshot/snapshot_017_pos.ksh \
+	functional/snapshot/snapshot_018_pos.ksh \
 	functional/snapused/cleanup.ksh \
 	functional/snapused/setup.ksh \
 	functional/snapused/snapused_001_pos.ksh \

tests/zfs-tests/tests/functional/snapshot/snapshot_018_pos.ksh

@@ -0,0 +1,127 @@
+#! /bin/ksh -p
+#
+# CDDL HEADER START
+#
+# The contents of this file are subject to the terms of the
+# Common Development and Distribution License (the "License").
+# You may not use this file except in compliance with the License.
+#
+# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+# or http://www.opensolaris.org/os/licensing.
+# See the License for the specific language governing permissions
+# and limitations under the License.
+#
+# When distributing Covered Code, include this CDDL HEADER in each
+# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+# If applicable, add the following below this CDDL HEADER, with the
+# fields enclosed by brackets "[]" replaced with your own identifying
+# information: Portions Copyright [yyyy] [name of copyright owner]
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright 2022 iXsystems, Inc.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/snapshot/snapshot.cfg
+
+#
+# DESCRIPTION:
+# Verify the functionality of snapshots_changed property
+#
+# STRATEGY:
+# 1. Create a pool
+# 2. Verify snapshots_changed property is NULL
+# 3. Create a filesystem
+# 4. Verify snapshots_changed property is NULL
+# 5. Create snapshots for all filesystems
+# 6. Verify snapshots_changed property shows correct time
+# 7. Unmount all filesystems
+# 8. Create a snapshot while unmounted
+# 9. Verify snapshots_changed
+# 10. Mount the filsystems
+# 11. Verify snapshots_changed
+# 12. Destroy the snapshots
+# 13. Verify snapshots_changed
+#
+
+function cleanup
+{
+	create_pool $TESTPOOL $DISKS
+}
+
+verify_runnable "both"
+
+log_assert "Verify snapshots_changed property"
+
+log_onexit cleanup
+
+snap_testpool="$TESTPOOL@v1"
+snap_testfsv1="$TESTPOOL/$TESTFS@v1"
+snap_testfsv2="$TESTPOOL/$TESTFS@v2"
+snapdir=".zfs/snapshot"
+
+# Create filesystems and check snapshots_changed is NULL
+create_pool $TESTPOOL $DISKS
+snap_changed_testpool=$(zfs get -H -o value -p snapshots_changed $TESTPOOL)
+log_must eval "[[ $snap_changed_testpool == - ]]"
+tpool_snapdir=$(get_prop mountpoint $TESTPOOL)/$snapdir
+log_must eval "[[ $(stat_mtime $tpool_snapdir) == 0 ]]"
+
+log_must zfs create $TESTPOOL/$TESTFS
+snap_changed_testfs=$(zfs get -H -o value -p snapshots_changed $TESTPOOL/$TESTFS)
+log_must eval "[[ $snap_changed_testfs == - ]]"
+tfs_snapdir=$(get_prop mountpoint $TESTPOOL/$TESTFS)/$snapdir
+log_must eval "[[ $(stat_mtime $tfs_snapdir) == 0 ]]"
+
+# Create snapshots for filesystems and check snapshots_changed reports correct time
+curr_time=$(date '+%s')
+log_must zfs snapshot $snap_testpool
+snap_changed_testpool=$(zfs get -H -o value -p snapshots_changed $TESTPOOL)
+log_must eval "[[ $snap_changed_testpool -ge $curr_time ]]"
+log_must eval "[[ $(stat_mtime $tpool_snapdir) ==  $snap_changed_testpool ]]"
+
+curr_time=$(date '+%s')
+log_must zfs snapshot $snap_testfsv1
+snap_changed_testfs=$(zfs get -H -o value -p snapshots_changed $TESTPOOL/$TESTFS)
+log_must eval "[[ $snap_changed_testfs -ge $curr_time ]]"
+log_must eval "[[ $(stat_mtime $tfs_snapdir) ==  $snap_changed_testfs ]]"
+
+# Unmount the filesystems and check snapshots_changed has correct value after unmount
+log_must zfs unmount $TESTPOOL/$TESTFS
+log_must eval "[[ $(zfs get -H -o value -p snapshots_changed $TESTPOOL/$TESTFS) == $snap_changed_testfs ]]"
+
+# Create snapshot while unmounted
+curr_time=$(date '+%s')
+log_must zfs snapshot $snap_testfsv2
+snap_changed_testfs=$(zfs get -H -o value -p snapshots_changed $TESTPOOL/$TESTFS)
+log_must eval "[[ $snap_changed_testfs -ge $curr_time ]]"
+
+log_must zfs unmount $TESTPOOL
+log_must eval "[[ $(zfs get -H -o value -p snapshots_changed $TESTPOOL) == $snap_changed_testpool ]]"
+
+# Mount back the filesystems and check snapshots_changed still has correct value
+log_must zfs mount $TESTPOOL
+log_must eval "[[ $(zfs get -H -o value -p snapshots_changed $TESTPOOL) == $snap_changed_testpool ]]"
+log_must eval "[[ $(stat_mtime $tpool_snapdir) ==  $snap_changed_testpool ]]"
+
+log_must zfs mount $TESTPOOL/$TESTFS
+log_must eval "[[ $(zfs get -H -o value -p snapshots_changed $TESTPOOL/$TESTFS) == $snap_changed_testfs ]]"
+log_must eval "[[ $(stat_mtime $tfs_snapdir) ==  $snap_changed_testfs ]]"
+
+# Destroy the snapshots and check snapshots_changed shows correct time
+curr_time=$(date '+%s')
+log_must zfs destroy $snap_testfsv1
+snap_changed_testfs=$(zfs get -H -o value -p snapshots_changed $TESTPOOL/$TESTFS)
+log_must eval "[[ $snap_changed_testfs -ge $curr_time ]]"
+log_must eval "[[ $(stat_mtime $tfs_snapdir) ==  $snap_changed_testfs ]]"
+
+curr_time=$(date '+%s')
+log_must zfs destroy $snap_testpool
+snap_changed_testpool=$(zfs get -H -o value -p snapshots_changed $TESTPOOL)
+log_must eval "[[ $snap_changed_testpool -ge $curr_time ]]"
+log_must eval "[[ $(stat_mtime $tpool_snapdir) ==  $snap_changed_testpool ]]"
+
+log_pass "snapshots_changed property behaves correctly"