Method and apparatus for expanding usable space for an application data file. A control file is maintained with control structures that indicate available and allocated portions of usable space in the data file, along with quantities of available space in portions of the data file. Access to the control structures is limited while the file is undergoing expansion. Space is allocated in the control file for new versions of control structures, and the contents of the control structures are copied to the space for the new versions of the control structures. Pointers that reference the control structures and that are maintained in the control file are updated to reference the new versions of the first and second control structures.