tape drive tape drive. Magnetic-tape drives with capacities of less than one megabyte were first used for data storage on
mainframe computers in the 1950s. , capacities of 20 terabytes or higher of uncompressed data per cartridge were available. In early computer systems, magnetic tape served as the main storage medium because although the drives were expensive, the tapes were inexpensive. Some computer systems ran the operating system on tape drives such as
DECtape. DECtape had fixed-size indexed blocks that could be rewritten without disturbing other blocks, so DECtape could be used like a slow disk drive. Data tape drives may use advanced data integrity techniques such as multilevel forward error correction, shingling, and
linear serpentine layout for writing data to tape. Tape drives can be connected to a computer with
SCSI,
Fibre Channel,
SATA,
USB,
FireWire,
FICON, or other interfaces. Tape drives are used with autoloaders and
tape libraries which automatically load, unload, and store multiple tapes, increasing the volume of data that can be stored without manual intervention. In the early days of
home computing,
floppy and
hard disk drives were very expensive. Many computers had an interface to store data via an audio
tape recorder, typically on
Compact Cassettes. Simple dedicated tape drives, such as the professional DECtape and the home
ZX Microdrive and
Rotronics Wafadrive, were also designed for inexpensive data storage. However, the drop in disk drive prices made such alternatives obsolete.
Data compression As some data can be
compressed to a smaller size than the original files, it has become commonplace when marketing tape drives to state the capacity with the assumption of a 2:1 compression ratio; thus a tape with a capacity of 80 GB would be sold as "80/160". The true storage capacity is also known as the
native capacity or the raw capacity. The compression ratio actually achievable depends on the data being compressed. Some data has little redundancy; large video files, for example, already use compression and cannot be compressed further. A
database with repetitive entries, on the other hand, may allow compression ratios better than 10:1.
Technical limitations 606 tape drive, showing two long vertical vacuum columns in the lower part.|alt=A large cabinet, about the size of an upright refrigerator, with a glass-covered top part holding two reels of magnetic tape, and a bottom part with control buttons framed by vertical channels. A disadvantageous effect termed '''''' occurs during read/write if the data transfer rate falls below the minimum threshold at which the tape drive heads were designed to transfer data to or from a continuously running tape. In this situation, the modern fast-running tape drive is unable to stop the tape instantly. Instead, the drive must decelerate and stop the tape, rewind it a short distance, restart it, position back to the point at which streaming stopped and then resume the operation. If the condition repeats, the resulting back-and-forth tape motion resembles that of
shining shoes with a cloth. Shoe-shining decreases the attainable data transfer rate, drive and tape life, and tape capacity. In early tape drives, non-continuous data transfer was normal and unavoidable. Computer processing power and available memory were usually insufficient to provide a constant stream, so tape drives were typically designed for
start-stop operation. Early drives used very large spools, which necessarily had high inertia and did not start and stop moving easily. To provide high start, stop and seek performance, several feet of loose tape was played out and pulled by a suction fan down into two deep open channels on either side of the
tape head and capstans. The long thin loops of tape hanging in these
vacuum columns had far less inertia than the two reels and could be rapidly started, stopped and repositioned. The large reels would move as required to keep the slack tape in the vacuum columns. Later, most tape drives of the 1980s introduced the use of an
internal data buffer to somewhat reduce start-stop situations. These drives are often referred to as
tape streamers. The tape was stopped only when
the buffer contained no data to be written, or when it was full of data during reading. As faster tape drives became available, despite being buffered, the drives started to suffer from the shoe-shining sequence of stop, rewind, start. Some newer drives have several speeds and implement algorithms that dynamically match the tape speed level to the computer's data rate. Example speed levels could be 50 percent, 75 percent and 100 percent of full speed. A computer that streams data slower than the lowest speed level (e.g., at 49 percent) will still cause shoe-shining. == Media ==