Update doc

docs/usage/writing/index.md

@@ -67,4 +67,8 @@ In this case, you can use a `predicate` to overwrite only the relevant records o
     Data written must conform to the same predicate, i.e. not contain any records that don't match the `predicate` condition, 
     otherwise the operation will fail 
 
-{{ code_example('operations', 'replace_where', ['replaceWhere'])}}
+{{ code_example('operations', 'replace_where', ['replaceWhere'])}}
+
+## Enable unsafe writes in local file system (opt-in)
+
+If the local file system does not support hard links, such as blobfuse, s3fs-fuse, you can choose to set the `FILE_ALLOW_UNSAFE_RENAME` variable to ``true`` in order to enable unsafe writes.