1. A method, comprising: updating a first index node representing a file and having a plurality of entries representing extents of the file, determining whether the updated first index node has exceeded a threshold number of extents; and responsive to determining that the updated first index node has exceeded the threshold number of extents: generating a copy-on-write (COW) B-tree data structure containing the plurality of entries representing the extents of the file; and modifying the first index node to point to the COW B-tree data structure.