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Magnetic Tape


Magnetic tape was first invented for recording sound by Fritz Pfleumer in 1928 in Germany, based on the invention of magnetic wire recording by Valdemar Poulsen in 1898. It is a non-volatile storage medium consisting of a magnetic coating on a thin plastic strip. Nearly all recording tape is of this type, whether used for video, audio storage or general purpose digital data storage using a computer. Magneto-optical and optical tape storage products have been developed using many of the same concepts as magnetic storage, but have achieved little commercial success. Magnetic tape was first used to record computer data in 1951 on the Mauchly-Eckert UNIVAC I. The recording medium was a 1/2 inch (13 mm) wide thin band of nickel-plated bronze. Recording density was 128 characters per inch (198 micrometre/character) on eight tracks at a linear speed of 100 in/s (2.54 m/s), yielding a data rate of 12,800 characters per second. Making allowance for the empty space between tape blocks, the actual transfer rate was around 7,200 characters per second.

Tape remains a viable alternative to disk due to its higher bit density and lower cost per bit. Tape has historically offered enough advantage in these two areas above disk storage to make it a viable product, particularly for backup. The rapid improvement in disk storage density and price (see Kryder's law), coupled with arguably less-vigorous innovation in tape storage, has reduced the market share of tape storage products.

In a typical format, data is written to tape in blocks with inter-block gaps between them, and each block is written in a single operation with the tape running continuously during the write. However, since the rate at which data is written or read to the tape drive is not deterministic, a tape drive usually has to cope with a difference between the rate at which data goes on and off the tape and the rate at which data is supplied or demanded by its host. A large memory buffer can be used to queue the data. The tape drive can be stopped, backed up, and restarted. The host can assist this process by choosing appropriate block sizes to send to the tape drive. There is a complex tradeoff between block size, the size of the data buffer in the record/playback deck, the percentage of tape lost on inter-block gaps, and read/write throughput.

Tape has quite a long data latency for random accesses since the deck must wind an average of 1/3 the tape length to move from one arbitrary data block to another. Most tape systems attempt to alleviate the intrinsic long latency, either using indexing, where a separate lookup table is maintained which gives the physical tape location for a given data block number, or by marking blocks with a tape mark that can be detected while winding the tape at high speed.

Most tape drives now include some kind of data compression.

The actual compression algorithms used are not the most effective known today, and better results can usually be obtained by turning off the compression built into the device and using a software compression program instead. Software compression also allows encryption to be performed after compression. However, software compression can place high loads on processors. Future tape drive will likely incorporate hardware encryption after the compression.