A peripheral unit that combines a disk cache and tape library to improve performance and eliminate cartridge waste. Also called a "virtual tape server," data intended for tape are written to the disk cache and later written to tape in the background. If a tape is read frequently, a certain amount of its data is held in the cache, which greatly improves access times over the slow loading process of a robotic library. Tape files, no matter how small, are typically stored on one cartridge. A virtual tape system (VTS) provides volume stacking, which stores multiple files on a single volume. The virtual tape system indexes the files and manages them, dramatically improving cartridge efficiency. See virtual tape library.