ZTS: ZIL txg sync fallback test

If the pool is degraded, ZIL writes should still succeed without falling
back to a full txg sync.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>

tests/runfiles/linux.run

@@ -125,7 +125,7 @@ tests = ['auto_offline_001_pos', 'auto_online_001_pos', 'auto_online_002_pos',
 tags = ['functional', 'fault']
 
 [tests/functional/flush:Linux]
-tests = ['zil_flush_error']
+tests = ['zil_flush_error', 'zil_flush_fallback']
 tags = ['functional', 'flush']
 
 [tests/functional/features/large_dnode:Linux]

tests/zfs-tests/tests/Makefile.am

@@ -1518,6 +1518,7 @@ nobase_dist_datadir_zfs_tests_tests_SCRIPTS += \
 	functional/features/large_dnode/setup.ksh \
 	functional/flush/cleanup.ksh \
 	functional/flush/zil_flush_error.ksh \
+	functional/flush/zil_flush_fallback.ksh \
 	functional/flush/setup.ksh \
 	functional/grow/grow_pool_001_pos.ksh \
 	functional/grow/grow_replicas_001_pos.ksh \

tests/zfs-tests/tests/functional/flush/zil_flush_fallback.ksh

@@ -0,0 +1,89 @@
+#!/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 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) 2024, Klara, Inc.
+#
+
+#
+# This tests that when a pool is degraded, ZIL writes/flushes are still done
+# directly, rather than falling back to a full txg flush.
+#
+
+. $STF_SUITE/include/libtest.shlib
+
+verify_runnable "global"
+
+function cleanup {
+	default_cleanup_noexit
+}
+
+log_onexit cleanup
+
+log_assert "ZIL writes go direct to the pool even when the pool is degraded"
+
+function zil_commit_count {
+  kstat zil | grep ^zil_commit_count | awk '{ print $3 }'
+}
+function zil_commit_error_count {
+  kstat zil | grep ^zil_commit_error_count | awk '{ print $3 }'
+}
+
+DISK1=${DISKS%% *}
+log_must default_mirror_setup_noexit $DISKS
+
+# get the current count of commits vs errors
+typeset -i c1=$(zil_commit_count)
+typeset -i e1=$(zil_commit_error_count)
+
+# force a single ZIL commit
+log_must dd if=/dev/zero of=/$TESTPOOL/file bs=128k count=1 conv=fsync
+
+# get the updated count of commits vs errors
+typeset -i c2=$(zil_commit_count)
+typeset -i e2=$(zil_commit_error_count)
+
+# degrade the pool
+log_must zpool offline -f $TESTPOOL $DISK1
+log_must wait_for_degraded $TESTPOOL
+
+# force another ZIL commit
+log_must dd if=/dev/zero of=/$TESTPOOL/file bs=128k count=1 conv=fsync
+
+# get counts again
+typeset -i c3=$(zil_commit_count)
+typeset -i e3=$(zil_commit_error_count)
+
+# repair the pool
+log_must zpool online $TESTPOOL $DISK1
+
+# when pool is in good health, a ZIL commit should go direct to the
+# pool and not fall back to a txg sync
+log_must test $(( $c2 - $c1 )) -eq 1
+log_must test $(( $e2 - $e1 )) -eq 0
+
+# when pool is degraded but still writeable, ZIL should still go direct
+# to the pull and not fall back
+log_must test $(( $c3 - $c2 )) -eq 1
+log_must test $(( $e3 - $e2 )) -eq 0
+
+log_pass "ZIL writes go direct to the pool even when the pool is degraded"