TAPE DRIVES




A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.

A tape drive provides sequential access storage, unlike a disk drive, which provides random access storage. A disk drive read/write head can move to any position on the disk in a few milliseconds, but a tape drive must physically wind tape between reels to read any one particular piece of data. As a result, tape drives have very slow average seek times. For sequential access once the tape is positioned, however, tape drives can stream data very fast. For example, as of 2010[update] Linear Tape-Open (LTO) supported continuous data transfer rates of up to 140 MB/s, comparable to hard disk drives.

Tape drives can range in capacity from a few megabytes to hundreds of gigabytes of uncompressed data.

As some data can be compressed to a smaller size than the files on hard disc, it has become commonplace when marketing taps 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 compression ratio actually achievable depends on the data being compressed. Some data has little redundancy; large video files, for example, already use compression technology and cannot be compressed further. A sparse database, on the other hand, may allow compression ratios better than 10:1.

Tape drives can be connected to a computer with SCSI (most common), 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 which can be stored without manual intervention.

Some older tape drives were designed as inexpensive alternatives to very expensive disk drives. Examples include DECtape, the ZX Microdrive and Rotronics Wafadrive. This is generally not feasible with modern tape drives that use advanced techniques like multilevel forward error correction, shingling, and serpentine layout for writing data to tape, and was made unnecessary by decreasing disk drive prices anyway.

Gartner Group estimated that 10 to 50 percent of all tape restores fail. Storage Magazine and Gartner reported that 34% of surveyed companies never test a restore from tape, and of those that do test, 77% experienced tape backup failures.

An effect referred to as shoe-shining may occur during read/write operations 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. When the transfer rate becomes too low and streaming is no longer possible, 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. The resulting back-and-forth tape motion resembles that of shining shoes with a cloth. Shoe-shining decreases the attainable data transfer rate, and drive and tape life.

In early tape drives, non-continuous data transfer was normal and unavoidable – computer processing power and memory available were usually insufficient to provide a constant stream. So tape drives were typically designed for so called 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 seeking 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 occasionally move to take up written tape and play out more blank tape into 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, buffering was not always possible, and the drives started to suffer from the shoe-shining sequence of stop, rewind, start.

Most recently, drives no longer operate at a single fixed linear speed, but have several speeds. Internally, they 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.

Magnetic tape is commonly housed in a casing known as a cassette or cartridge—for example, the 4-track cartridge and the compact cassette. The cassette contains magnetic tape to provide different audio content using the same player. The outer shell, made of plastic, sometimes with metal plates and parts, permits ease of handling of the fragile tape, making it far more convenient and robust than having spools of exposed tape. Simple Compact Cassette audio tape recorders were commonly used for data storage and distribution on home computers at a time when floppy disk drives were very expensive. The Commodore Datassette was a dedicated data version of the same system.

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