diff --git a/mkfs.c b/mkfs.c
index 72ee2904..0ea39f38 100644
--- a/mkfs.c
+++ b/mkfs.c
@@ -926,7 +926,7 @@ fail_no_dir:
 static int open_target(char *output_name)
 {
 	int output_fd;
-	output_fd = open(output_name, O_CREAT | O_RDWR | O_TRUNC,
+	output_fd = open(output_name, O_CREAT | O_RDWR,
 		         S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH);
 
 	return output_fd;
diff --git a/tests/mkfs-tests/004-rootdir-keeps-size/test.sh b/tests/mkfs-tests/004-rootdir-keeps-size/test.sh
new file mode 100755
index 00000000..a78a3dec
--- /dev/null
+++ b/tests/mkfs-tests/004-rootdir-keeps-size/test.sh
@@ -0,0 +1,30 @@
+#!/bin/bash
+# make sure that mkfs.btrfs --rootsize does not change size of the image
+
+source $TOP/tests/common
+
+check_prereq mkfs.btrfs
+prepare_test_dev
+
+test_mkfs_with_size() {
+	local size
+	local imgsize
+	local tmp
+
+	size="$1"
+	run_check truncate -s$size $TEST_DEV
+	imgsize=$(run_check_stdout stat --format=%s $TEST_DEV)
+	run_check $SUDO_HELPER $TOP/mkfs.btrfs -f \
+		--rootdir $TOP/Documentation \
+		$TEST_DEV
+	tmp=$(run_check_stdout stat --format=%s $TEST_DEV)
+	if ! [ "$imgsize" = "$tmp" ]; then
+		_fail "image size changed from $imgsize to $tmp"
+	fi
+}
+
+test_mkfs_with_size 128M
+test_mkfs_with_size 256M
+test_mkfs_with_size 512M
+test_mkfs_with_size 1G
+test_mkfs_with_size 2G