Add BRT support to zpool prefetch command

Implement BRT (Block Reference Table) prefetch functionality similar
to existing DDT prefetch.  This allows preloading BRT metadata into
ARC to improve performance for block cloning operations and frees
of earlier cloned blocks.

Make -t parameter optional.  When omitted, prefetch all supported
metadata types (both DDT and BRT now).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17890

tests/zfs-tests/tests/Makefile.am

@@ -1217,6 +1217,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	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_prefetch/zpool_prefetch_002_pos.ksh \
 	functional/cli_root/zpool_reguid/cleanup.ksh \
 	functional/cli_root/zpool_reguid/setup.ksh \
 	functional/cli_root/zpool_reguid/zpool_reguid_001_pos.ksh \

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

@@ -42,6 +42,15 @@ verify_runnable "both"
 
 log_assert "'zpool prefetch -t ddt <pool>' can successfully load the DDT for a pool."
 
+DATASET=$TESTPOOL/ddt
+
+function cleanup
+{
+	datasetexists $DATASET && destroy_dataset $DATASET -f
+}
+
+log_onexit cleanup
+
 function getddtstats
 {
 	typeset -n gds=$1
@@ -75,9 +84,8 @@ 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 compression=off -o dedup=on $DATASET
-MNTPOINT=$(get_prop mountpoint $TESTPOOL/ddt)
+MNTPOINT=$(get_prop mountpoint $DATASET)
 
 log_note "Generating dataset ..."
 typeset -i i=0

tests/zfs-tests/tests/functional/cli_root/zpool_prefetch/zpool_prefetch_002_pos.ksh

@@ -0,0 +1,95 @@
+#!/bin/ksh -p
+# SPDX-License-Identifier: CDDL-1.0
+#
+# 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) 2025 by iXsystems, Inc.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+#
+# DESCRIPTION:
+# 'zpool prefetch -t brt <pool>' can successfully load a pool's BRT on demand.
+# 'zpool prefetch <pool>' without -t prefetches both DDT and BRT.
+#
+# STRATEGY:
+# 1. Create a dataset with block cloning enabled.
+# 2. Create files and clone them to populate the BRT.
+# 3. Export and import the pool to flush caches.
+# 4. Use zpool prefetch -t brt to load BRT.
+# 5. Test zpool prefetch without -t to prefetch all types.
+#
+
+verify_runnable "both"
+
+if ! command -v clonefile > /dev/null ; then
+	log_unsupported "clonefile program required to test block cloning"
+fi
+
+log_assert "'zpool prefetch' can successfully load BRT and prefetch all types"
+
+DATASET=$TESTPOOL/brt
+
+function cleanup
+{
+	datasetexists $DATASET && destroy_dataset $DATASET -f
+}
+
+log_onexit cleanup
+log_must zfs create $DATASET
+MNTPOINT=$(get_prop mountpoint $DATASET)
+
+log_note "Generating cloned blocks for BRT ..."
+
+# Create source file
+log_must dd if=/dev/urandom of=$MNTPOINT/source bs=1M count=100
+
+# Create clones using clonefile
+typeset -i i=0
+while (( i < 50 )); do
+	log_must clonefile -f $MNTPOINT/source $MNTPOINT/clone.$i
+	((i += 1))
+done
+
+sync_pool $TESTPOOL
+
+# Verify BRT has entries (non-zero saved space)
+brt_saved=$(zpool get -Hp -o value bclone_saved $TESTPOOL)
+log_note "BRT saved space: $brt_saved"
+log_must test "$brt_saved" -gt "0"
+
+# Export/import to flush caches
+log_must zpool export $TESTPOOL
+log_must zpool import $TESTPOOL
+
+# Test BRT prefetch - verify command succeeds
+# Note: BRT does not expose cache statistics like DDT, so we can only
+# verify the prefetch command completes successfully
+log_must zpool prefetch -t brt $TESTPOOL
+
+# Test prefetch without -t (should prefetch all types including BRT)
+log_must zpool export $TESTPOOL
+log_must zpool import $TESTPOOL
+log_must zpool prefetch $TESTPOOL
+
+log_pass "'zpool prefetch' successfully loads BRT and all types"