ddt: add support for prefetching tables into the ARC

This change adds a new `zpool prefetch -t ddt $pool` command which
causes a pool's DDT to be loaded into the ARC. The primary goal is to
remove the need to "warm" a pool's cache before deduplication stops
slowing write performance. It may also provide a way to reload portions
of a DDT if they have been flushed due to inactivity.

Sponsored-by: iXsystems, Inc.
Sponsored-by: Catalogics, Inc.
Sponsored-by: Klara, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Will Andrews <will.andrews@klarasystems.com>
Signed-off-by: Fred Weigel <fred.weigel@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Co-authored-by: Will Andrews <will.andrews@klarasystems.com>
Co-authored-by: Don Brady <don.brady@klarasystems.com>
Closes #15890

tests/runfiles/common.run

@@ -208,6 +208,10 @@ tests = ['zfs_create_001_pos', 'zfs_create_002_pos', 'zfs_create_003_pos',
     'zfs_create_verbose']
 tags = ['functional', 'cli_root', 'zfs_create']
 
+[tests/functional/cli_root/zpool_prefetch]
+tests = ['zpool_prefetch_001_pos']
+tags = ['functional', 'cli_root', 'zpool_prefetch']
+
 [tests/functional/cli_root/zfs_destroy]
 tests = ['zfs_clone_livelist_condense_and_disable',
     'zfs_clone_livelist_condense_races', 'zfs_clone_livelist_dedup',

tests/zfs-tests/tests/Makefile.am

@@ -1176,6 +1176,9 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/cli_root/zpool_online/setup.ksh \
 	functional/cli_root/zpool_online/zpool_online_001_pos.ksh \
 	functional/cli_root/zpool_online/zpool_online_002_neg.ksh \
+	functional/cli_root/zpool_prefetch/cleanup.ksh \
+	functional/cli_root/zpool_prefetch/setup.ksh \
+	functional/cli_root/zpool_prefetch/zpool_prefetch_001_pos.ksh \
 	functional/cli_root/zpool_remove/cleanup.ksh \
 	functional/cli_root/zpool_remove/setup.ksh \
 	functional/cli_root/zpool_remove/zpool_remove_001_neg.ksh \

tests/zfs-tests/tests/functional/cli_root/zpool_prefetch/cleanup.ksh

@@ -0,0 +1,30 @@
+#!/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 2007 Sun Microsystems, Inc.  All rights reserved.
+# Use is subject to license terms.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+default_cleanup

tests/zfs-tests/tests/functional/cli_root/zpool_prefetch/setup.ksh

@@ -0,0 +1,32 @@
+#!/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 2007 Sun Microsystems, Inc.  All rights reserved.
+# Use is subject to license terms.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+DISK=${DISKS%% *}
+
+default_setup $DISK

tests/zfs-tests/tests/functional/cli_root/zpool_prefetch/zpool_prefetch_001_pos.ksh

@@ -0,0 +1,128 @@
+#!/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 (c) 2019, 2023 by Klara Inc.  All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# 'zpool prefetch -t ddt <pool>' can successfully load a pool's DDT on demand.
+#
+# STRATEGY:
+# 1. Build up storage pool with deduplicated dataset.
+# 2. Export the pool.
+# 3. Import the pool, and use zpool prefetch -t ddt to load its table.
+# 4. Verify the DDT was loaded successfully using ddt cache stats
+#
+
+verify_runnable "both"
+
+log_assert "'zpool prefetch -t ddt <pool>' can successfully load the DDT for a pool."
+
+function getddtstats
+{
+	typeset -n gds=$1
+	typeset pool=$2
+
+	out=$(zpool status -DDp $pool | awk '/^ dedup: / {print $6 " " $9 " " $12}')
+	log_note "status -DDp output: ${out}"
+
+	gds.ondisk=$(echo $out | cut -d" " -f1)
+	gds.incore=$(echo $out | cut -d" " -f2)
+	gds.cached=$(echo $out | cut -d" " -f3)
+
+	# In case of missing data, reset to 0.  This should normally be due
+	# to a pool without any DDT.
+	[ -z "${gds.ondisk}" ] && gds.ondisk="0"
+	[ -z "${gds.incore}" ] && gds.incore="0"
+	[ -z "${gds.cached}" ] && gds.cached="0"
+
+	return true
+}
+
+# Confirm that nothing happens on a standard pool config.
+typeset -A before
+log_must getddtstats before $TESTPOOL
+log_note "before stats: ${before}"
+log_must test "${before.ondisk}" -eq "0"
+log_must test "${before.incore}" -eq "0"
+log_must test "${before.cached}" -eq "0"
+log_must zpool prefetch -t ddt $TESTPOOL
+
+# Build up the deduplicated dataset.  This consists of creating enough files
+# to generate a reasonable size DDT for testing purposes.
+
+DATASET=$TESTPOOL/ddt
+log_must zfs create -o dedup=on $DATASET
+MNTPOINT=$(get_prop mountpoint $TESTPOOL/ddt)
+
+log_note "Generating dataset ..."
+typeset -i i=0
+while (( i < 16384 )); do
+	echo -n $i > $MNTPOINT/f.$i
+
+	# Create some copies of the original mainly for the purpose of
+	# having duplicate entries.  About half will have no copies, while
+	# the remainder will have an equal distribution of 1-4 copies,
+	# depending on the number put into the original.
+	typeset -i j
+	((j = i % 8))
+	while (( j < 4 )); do
+		cp $MNTPOINT/f.$i $MNTPOINT/f.$i.$j
+		((j += 1))
+	done
+	((i += 1))
+done
+log_note "Dataset generation completed."
+
+typeset -A generated
+log_must getddtstats generated $TESTPOOL
+log_note "generated stats: ${generated}"
+log_must test "${generated.ondisk}" -ge "1048576"
+log_must test "${generated.incore}" -ge "1048576"
+log_must test "${generated.cached}" -ge "1048576"
+log_must zpool prefetch -t ddt $TESTPOOL
+
+# Do an export/import series to flush the DDT dataset cache.
+typeset -A reimport
+log_must zpool export $TESTPOOL
+log_must zpool import $TESTPOOL
+log_must getddtstats reimport $TESTPOOL
+log_note "reimport stats: ${reimport}"
+log_must test "${reimport.ondisk}" -ge "1048576"
+log_must test "${reimport.incore}" -ge "1048576"
+# On reimport, only the first block or two should be cached.
+log_must test "${reimport.cached}" -le "65536"
+
+# Finally, reload it and check again.
+typeset -A reloaded
+log_must zpool prefetch -t ddt $TESTPOOL
+log_must getddtstats reloaded $TESTPOOL
+log_note "reloaded stats: ${reloaded}"
+log_must test "${reloaded.ondisk}" -ge "1048576"
+log_must test "${reloaded.incore}" -ge "1048576"
+log_must test "${reloaded.cached}" -eq "${reloaded.incore}"
+
+log_pass "'zpool prefetch -t ddt <pool>' success."