diff --git a/tests/zfs-tests/tests/perf/regression/random_readwrite.ksh b/tests/zfs-tests/tests/perf/regression/random_readwrite.ksh
index 3a358774f565..e0626c0b42f3 100755
--- a/tests/zfs-tests/tests/perf/regression/random_readwrite.ksh
+++ b/tests/zfs-tests/tests/perf/regression/random_readwrite.ksh
@@ -1,99 +1,99 @@
 #!/bin/ksh
 
 #
 # This file and its contents are supplied under the terms of the
 # Common Development and Distribution License ("CDDL"), version 1.0.
 # You may only use this file in accordance with the terms of version
 # 1.0 of the CDDL.
 #
 # A full copy of the text of the CDDL should have accompanied this
 # source.  A copy of the CDDL is also available via the Internet at
 # http://www.illumos.org/license/CDDL.
 #
 
 #
 # Copyright (c) 2015, 2021 by Delphix. All rights reserved.
 #
 
 #
 # Description:
 # Trigger fio runs using the random_readwrite job file. The number of runs and
 # data collected is determined by the PERF_* variables. See do_fio_run for
 # details about these variables.
 #
 # The files to read and write from are created prior to the first fio run,
 # and used for all fio runs. The ARC is cleared with `zinject -a` prior to
 # each run so reads will go to disk.
 #
 # Thread/Concurrency settings:
 #    PERF_NTHREADS defines the number of files created in the test filesystem,
 #    as well as the number of threads that will simultaneously drive IO to
 #    those files.  The settings chosen are from measurements in the
 #    PerfAutoESX/ZFSPerfESX Environments, selected at concurrency levels that
 #    are at peak throughput but lowest latency.  Higher concurrency introduces
 #    queue time latency and would reduce the impact of code-induced performance
 #    regressions.
 #
 
 . $STF_SUITE/include/libtest.shlib
 . $STF_SUITE/tests/perf/perf.shlib
 
 command -v fio > /dev/null || log_unsupported "fio missing"
 
 function cleanup
 {
 	# kill fio and iostat
 	pkill fio
 	pkill iostat
 	recreate_perf_pool
 }
 
 trap "log_fail \"Measure IO stats during random read load\"" SIGTERM
 log_onexit cleanup
 
 recreate_perf_pool
 populate_perf_filesystems
 
 # Aim to fill the pool to 50% capacity while accounting for a 3x compressratio.
 export TOTAL_SIZE=$(($(get_prop avail $PERFPOOL) * 3 / 2))
 
 # Variables specific to this test for use by fio.
 export PERF_NTHREADS=${PERF_NTHREADS:-'32 64'}
 export PERF_NTHREADS_PER_FS=${PERF_NTHREADS_PER_FS:-'0'}
-export PERF_IOSIZES=''		# bssplit used instead
+export PERF_IOSIZES='bssplit'		# bssplit used instead of fixed sizes
 export PERF_SYNC_TYPES=${PERF_SYNC_TYPES:-'1'}
 
 # Layout the files to be used by the readwrite tests. Create as many files
 # as the largest number of threads. An fio run with fewer threads will use
 # a subset of the available files.
 export NUMJOBS=$(get_max $PERF_NTHREADS)
 export FILE_SIZE=$((TOTAL_SIZE / NUMJOBS))
 export DIRECTORY=$(get_directory)
 log_must fio $FIO_SCRIPTS/mkfiles.fio
 
 # Set up the scripts and output files that will log performance data.
 lun_list=$(pool_to_lun_list $PERFPOOL)
 log_note "Collecting backend IO stats with lun list $lun_list"
 if is_linux; then
 	typeset perf_record_cmd="perf record -F 99 -a -g -q \
 	    -o /dev/stdout -- sleep ${PERF_RUNTIME}"
 
 	export collect_scripts=(
 	    "zpool iostat -lpvyL $PERFPOOL 1" "zpool.iostat"
 	    "vmstat -t 1" "vmstat"
 	    "mpstat -P ALL 1" "mpstat"
 	    "iostat -tdxyz 1" "iostat"
 	    "$perf_record_cmd" "perf"
 	)
 else
 	export collect_scripts=(
 	    "$PERF_SCRIPTS/io.d $PERFPOOL $lun_list 1" "io"
 	    "vmstat -T d 1" "vmstat"
 	    "mpstat -T d 1" "mpstat"
 	    "iostat -T d -xcnz 1" "iostat"
 	)
 fi
 
 log_note "Random reads and writes with settings: $(print_perf_settings)"
 do_fio_run random_readwrite.fio false true
 log_pass "Measure IO stats during random read and write load"