A file system uses a B-tree data structure to organize file data. The file system may maintain an index node (mode) representing a file and having entries that map to extents of the file. When the file system detects an index node, through updates, has exceeded a threshold number of extents, the file system converts the file to a copy-on-write (COW) B-tree data structure containing the entries representing the extents of the file. To clone the file, the file system uses copies of the index node and the root node of the COW B-tree data structure.