One additional aspect of demand paging is the handling and overall use of swap space. Disk I/O to swap space is generally faster than that to the file system. It is faster because swap space is allocated in much larger blocks, and file lookups and indirect allocation methods are not used. It is therefore possible for the system to gain better paging throughput, by copying an entire file demand paging from the swap space. Systems with limited swap space can still employ such a scheme when binary file are used. Demand pages for such files are brought for, these pages can simply be overwritten (since they are never modified) and read in from the file system again if needed. Yet another option is initially to demand pages from the file system, but to write the pages to swap space as they are ever read from the file system, but all subsequent paging is done from swap space. This method appears to be the optimum one. it is used in BSD UNIX