diff --git a/cmd/zhack.c b/cmd/zhack.c
index fe1c697e1..edf9dfa2c 100644
--- a/cmd/zhack.c
+++ b/cmd/zhack.c
@@ -70,7 +70,8 @@ static __attribute__((noreturn)) void
 usage(void)
 {
 	(void) fprintf(stderr,
-	    "Usage: zhack [-c cachefile] [-d dir] <subcommand> <args> ...\n"
+	    "Usage: zhack [-o tunable] [-c cachefile] [-d dir] <subcommand> "
+	    "<args> ...\n"
 	    "where <subcommand> <args> is one of the following:\n"
 	    "\n");
 
@@ -1169,7 +1170,7 @@ main(int argc, char **argv)
 	dprintf_setup(&argc, argv);
 	zfs_prop_init();
 
-	while ((c = getopt(argc, argv, "+c:d:")) != -1) {
+	while ((c = getopt(argc, argv, "+c:d:o:")) != -1) {
 		switch (c) {
 		case 'c':
 			g_importargs.cachefile = optarg;
@@ -1178,6 +1179,10 @@ main(int argc, char **argv)
 			assert(g_importargs.paths < MAX_NUM_PATHS);
 			g_importargs.path[g_importargs.paths++] = optarg;
 			break;
+		case 'o':
+			if (handle_tunable_option(optarg, B_FALSE) != 0)
+				exit(1);
+			break;
 		default:
 			usage();
 			break;
diff --git a/include/sys/zfs_file.h b/include/sys/zfs_file.h
index a1f344c2b..67abe9988 100644
--- a/include/sys/zfs_file.h
+++ b/include/sys/zfs_file.h
@@ -46,7 +46,7 @@ void zfs_file_close(zfs_file_t *fp);
 
 int zfs_file_write(zfs_file_t *fp, const void *buf, size_t len, ssize_t *resid);
 int zfs_file_pwrite(zfs_file_t *fp, const void *buf, size_t len, loff_t off,
-    ssize_t *resid);
+    uint8_t ashift, ssize_t *resid);
 int zfs_file_read(zfs_file_t *fp, void *buf, size_t len, ssize_t *resid);
 int zfs_file_pread(zfs_file_t *fp, void *buf, size_t len, loff_t off,
     ssize_t *resid);
diff --git a/lib/libzpool/kernel.c b/lib/libzpool/kernel.c
index fea2f8145..8ed374627 100644
--- a/lib/libzpool/kernel.c
+++ b/lib/libzpool/kernel.c
@@ -1238,7 +1238,7 @@ zfs_file_write(zfs_file_t *fp, const void *buf, size_t count, ssize_t *resid)
  */
 int
 zfs_file_pwrite(zfs_file_t *fp, const void *buf,
-    size_t count, loff_t pos, ssize_t *resid)
+    size_t count, loff_t pos, uint8_t ashift, ssize_t *resid)
 {
 	ssize_t rc, split, done;
 	int sectors;
@@ -1248,8 +1248,8 @@ zfs_file_pwrite(zfs_file_t *fp, const void *buf,
 	 * system calls so that the process can be killed in between.
 	 * This is used by ztest to simulate realistic failure modes.
 	 */
-	sectors = count >> SPA_MINBLOCKSHIFT;
-	split = (sectors > 0 ? rand() % sectors : 0) << SPA_MINBLOCKSHIFT;
+	sectors = count >> ashift;
+	split = (sectors > 0 ? rand() % sectors : 0) << ashift;
 	rc = pwrite64(fp->f_fd, buf, split, pos);
 	if (rc != -1) {
 		done = rc;
diff --git a/man/man1/zhack.1 b/man/man1/zhack.1
index ebb136477..63658cf93 100644
--- a/man/man1/zhack.1
+++ b/man/man1/zhack.1
@@ -161,6 +161,8 @@ Search for
 members in
 .Ar dir .
 Can be specified more than once.
+.It Fl o Ar var Ns = Ns Ar value
+Set the given tunable to the provided value.
 .El
 .
 .Sh EXAMPLES
diff --git a/module/os/freebsd/zfs/zfs_file_os.c b/module/os/freebsd/zfs/zfs_file_os.c
index 21e5f7938..ca13569a1 100644
--- a/module/os/freebsd/zfs/zfs_file_os.c
+++ b/module/os/freebsd/zfs/zfs_file_os.c
@@ -164,8 +164,9 @@ zfs_file_write(zfs_file_t *fp, const void *buf, size_t count, ssize_t *resid)
 
 int
 zfs_file_pwrite(zfs_file_t *fp, const void *buf, size_t count, loff_t off,
-    ssize_t *resid)
+    uint8_t ashift, ssize_t *resid)
 {
+	(void) ashift;
 	return (zfs_file_write_impl(fp, buf, count, &off, resid));
 }
 
diff --git a/module/os/linux/zfs/zfs_file_os.c b/module/os/linux/zfs/zfs_file_os.c
index c72994736..3fdcdbac6 100644
--- a/module/os/linux/zfs/zfs_file_os.c
+++ b/module/os/linux/zfs/zfs_file_os.c
@@ -115,8 +115,9 @@ zfs_file_write(zfs_file_t *fp, const void *buf, size_t count, ssize_t *resid)
  */
 int
 zfs_file_pwrite(zfs_file_t *fp, const void *buf, size_t count, loff_t off,
-    ssize_t *resid)
+    uint8_t ashift, ssize_t *resid)
 {
+	(void) ashift;
 	ssize_t rc;
 
 	rc  = kernel_write(fp, buf, count, &off);
diff --git a/module/zfs/vdev_file.c b/module/zfs/vdev_file.c
index f457669bc..20b4db65e 100644
--- a/module/zfs/vdev_file.c
+++ b/module/zfs/vdev_file.c
@@ -228,7 +228,8 @@ vdev_file_io_strategy(void *arg)
 		abd_return_buf_copy(zio->io_abd, buf, size);
 	} else {
 		buf = abd_borrow_buf_copy(zio->io_abd, zio->io_size);
-		err = zfs_file_pwrite(vf->vf_file, buf, size, off, &resid);
+		err = zfs_file_pwrite(vf->vf_file, buf, size, off,
+		    vd->vdev_ashift, &resid);
 		abd_return_buf(zio->io_abd, buf, size);
 	}
 	zio->io_error = err;