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The IBM Virtual Tape Server provides a revolutionary tape storage solution that emulates tape drives using a disk cache, providing up to 256 virtual tape-drive images and stacking the virtual tape volumes onto a maximum of 12 physical tape drives. The high ratio of virtual to physical drives, combined with the stacking of virtual volumes, provides tape storage that is both cost- and space-efficient. Existing IBM products are used as microcode building blocks to provide the core functions of the virtual tape server. These core functions are integrated into a reliable, high-performance storage subsystem by autonomic computing controls.
Introduction
The year 2002 marked IBM's 50th year in the computer tape business, a half-century that began with the introduction of the IBM 726 magnetic tape drive [1]. Initially, tape drives were directly connected to their host computers. In 1984, IBM developed the first buffering tape-control units, which attached between host computers and tape drives. These control units provided improved performance, protocol conversion, and improved error handling. As both control-unit and tape-drive performance improved, it became apparent that many applications were unable to make efficient use of the bandwidth provided by the tape drives.
At that time, IBM analyzed the way in which mainframe customers were using tape drives. Although customers were able to use the full tape-cartridge capacity in some applications, most applications were using smaller volume sizes that left a large percentage of physical tape-cartridge capacity empty. It was determined that IBM customers had an uncompressed average tape-volume size of only 250 MB at a time when tape capacity was approaching 10 GB per cartridge. In addition, many of the applications customers were running could make use of less than 1 MB/s of the tape-drive bandwidth.
Only a small minority of customers were fully utilizing the capacity and throughput provided by IBM high-end tape drives. The majority of IBM customers were using tape applications in a manner that prevented them from taking full advantage of the throughput of IBM tape drives and the capacity of IBM tapes. In fact, according to an estimate by IBM and the research firm International Data Corporation (IDC), of the customers using IBM 3490 technology, only approximately "10% of each data center's tape cartridge is filled with data ....