The optical disk system is a hierarchical system with primary storage on magnetic disk and secondary storage on the optical disks. New files are written to the write cache. Periodically the optical disk archiver copies the files from the write cache to the optical disk. This process also copies the files to the read cache and deletes them from the write cache. The PPPL utilities and libraries that access this data first check the magnetic cache. If the file is not found, the optical disk indices are searched; if the file is found, it is copied to the read cache. The data on the read cache is skimmed by access date.