reproducer.sh

#!/bin/bash
#
# Run this script multiple times in parallel inside your pool's mount
# to reproduce https://github.com/openzfs/zfs/issues/15526.  Like:
#
# ./reproducer.sh & ./reproducer.sh & ./reproducer.sh & /reproducer.sh & wait
#

if [ $(cat /sys/module/zfs/parameters/zfs_bclone_enabled) != "1" ] ; then
	echo "please set /sys/module/zfs/parameters/zfs_bclone_enabled = 1"
	exit
fi

prefix="reproducer_${BASHPID}_"
dd if=/dev/urandom of=${prefix}0 bs=1M count=1 status=none

echo "writing files"
end=1000
h=0
for i in `seq 1 2 $end` ; do
	let "j=$i+1"
	cp  ${prefix}$h ${prefix}$i
	cp --reflink=never ${prefix}$i ${prefix}$j
	let "h++"
done

echo "checking files"
for i in `seq 1 $end` ; do
	diff ${prefix}0 ${prefix}$i
done

Illumos version is in https://www.illumos.org/issues/16087#note-6