Implement physical rewrites

Based on previous commit this implements `zfs rewrite -P` flag,
making ZFS to keep blocks logical birth times while rewriting
files.  It should exclude the rewritten blocks from incremental
sends, snapshot diffs, etc.  Snapshots space usage same time will
reflect the additional space usage from newly allocated blocks.

Since this begins to use new "rewrite" flag in the block pointers,
this commit introduces a new read-compatible per-dataset feature
physical_rewrite.  It must be enabled for the command to not fail,
it is activated on first use and deactivated on deletion of the
last affected dataset.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:  Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17565

tests/zfs-tests/tests/Makefile.am

@@ -869,6 +869,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/cli_root/zfs_rewrite/cleanup.ksh \
 	functional/cli_root/zfs_rewrite/setup.ksh \
 	functional/cli_root/zfs_rewrite/zfs_rewrite.ksh \
+	functional/cli_root/zfs_rewrite/zfs_rewrite_physical.ksh \
 	functional/cli_root/zfs_rollback/cleanup.ksh \
 	functional/cli_root/zfs_rollback/setup.ksh \
 	functional/cli_root/zfs_rollback/zfs_rollback_001_pos.ksh \

tests/zfs-tests/tests/functional/cli_root/zfs_rewrite/zfs_rewrite_physical.ksh

@@ -0,0 +1,100 @@
+#!/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 https://opensource.org/licenses/CDDL-1.0.
+# 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, iXsystems, Inc.
+#
+
+# DESCRIPTION:
+#	Verify zfs rewrite -P flag correctly preserves logical birth times.
+#
+# STRATEGY:
+#	1. Create a test file and sync it.
+#	2. Create a snapshot to capture the original birth time.
+#	3. Test default rewrite behavior (updates logical birth time).
+#	4. Test -P flag behavior (preserves logical birth time).
+#	5. Verify incremental send behavior difference.
+
+. $STF_SUITE/include/libtest.shlib
+
+typeset tmp=$(mktemp)
+typeset send_default=$(mktemp)
+typeset send_physical=$(mktemp)
+
+function cleanup
+{
+	rm -rf $tmp $send_default $send_physical $TESTDIR/*
+	zfs destroy -R $TESTPOOL/$TESTFS@snap1 2>/dev/null || true
+	zfs destroy -R $TESTPOOL/$TESTFS@snap2 2>/dev/null || true
+	zfs destroy -R $TESTPOOL/$TESTFS@snap3 2>/dev/null || true
+}
+
+log_assert "zfs rewrite -P flag correctly preserves logical birth times"
+
+log_onexit cleanup
+
+log_must zfs set recordsize=128k $TESTPOOL/$TESTFS
+
+# Create test file and initial snapshot
+log_must dd if=/dev/urandom of=$TESTDIR/testfile bs=128k count=4
+log_must sync_pool $TESTPOOL
+typeset orig_hash=$(xxh128digest $TESTDIR/testfile)
+log_must zfs snapshot $TESTPOOL/$TESTFS@snap1
+
+# Test default rewrite behavior (updates logical birth time)
+log_must zfs rewrite $TESTDIR/testfile
+log_must sync_pool $TESTPOOL
+typeset default_hash=$(xxh128digest $TESTDIR/testfile)
+log_must [ "$orig_hash" = "$default_hash" ]
+log_must zfs snapshot $TESTPOOL/$TESTFS@snap2
+
+# Test incremental send size - should be large with updated birth time
+log_must eval "zfs send -i @snap1 $TESTPOOL/$TESTFS@snap2 > $send_default"
+typeset default_size=$(wc -c < $send_default)
+log_note "Default rewrite incremental send size: $default_size bytes"
+
+# Reset the file to original state
+log_must zfs rollback -r $TESTPOOL/$TESTFS@snap1
+
+# Test -P flag behavior (preserves logical birth time)
+log_must zfs rewrite -P $TESTDIR/testfile
+log_must sync_pool $TESTPOOL
+typeset physical_hash=$(xxh128digest $TESTDIR/testfile)
+log_must [ "$orig_hash" = "$physical_hash" ]
+log_must zfs snapshot $TESTPOOL/$TESTFS@snap3
+
+# Test incremental send size - should be minimal with preserved birth time
+log_must eval "zfs send -i @snap1 $TESTPOOL/$TESTFS@snap3 > $send_physical"
+typeset physical_size=$(wc -c < $send_physical)
+log_note "Physical rewrite incremental send size: $physical_size bytes"
+
+# Verify that -P flag produces smaller incremental send
+if [[ $physical_size -lt $default_size ]]; then
+	log_note "SUCCESS: -P flag produces smaller incremental send" \
+	    "($physical_size < $default_size)"
+else
+	log_fail "FAIL: -P flag should produce smaller incremental send" \
+	    "($physical_size >= $default_size)"
+fi
+
+log_pass

tests/zfs-tests/tests/functional/cli_root/zpool_get/zpool_get.cfg

@@ -92,6 +92,7 @@ typeset -a properties=(
     "feature@draid"
     "feature@redaction_list_spill"
     "feature@dynamic_gang_header"
+    "feature@physical_rewrite"
 )
 
 if is_linux || is_freebsd; then