A method and apparatus for managing data structures associated with large files are provided. With the apparatus and method, region control blocks for a data file are allocated on demand from an array of control blocks and are organized into a tree data structure. The tree data structure includes a base region control block and zero or more extended region control blocks. The base region control block includes a pointer to a root of the tree data structure and information about the data file. The extended region control blocks have pointers to the base region control block, to a successor region control block, and pointers to zero to four child region control blocks. Using this tree data structure, the operating system may perform various operations on the region control blocks with improved performance.