diff --git a/tests/generic/420 b/tests/generic/420
index ccb6cb8e..aab2e088 100755
--- a/tests/generic/420
+++ b/tests/generic/420
@@ -52,10 +52,10 @@ _supported_os Linux
 _require_test
 _require_xfs_io_command fpunch
 
-testfile="${TEST_DIR}/testfile"
+testfile="${TEST_DIR}/testfile.$seq"
 
 echo "Testing fallocate(mode=FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE)"
-$XFS_IO_PROG -fc "pwrite -b 2048 0 2048" $testfile | head -n1
+$XFS_IO_PROG -f -t -c "pwrite -b 2048 0 2048" $testfile | head -n1
 # fpunch from xfs_io has KEEP_SIZE flag set
 $XFS_IO_PROG -c "fpunch 2048 2048" $testfile
 $XFS_IO_PROG -c "stat" $testfile | grep -F 'stat.size'