Tape drive

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A tape drive is a data storage device that reads and writes data on a magnetic tape. It 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 can move its read/write head(s) to any random part of the disk in a very short amount of time, but a tape drive must spend a considerable amount of time winding tape between reels to read any one particular piece of data. As a result, tape drives have very slow average seek times. Despite the slow seek time, tape drives can stream data to and from tape very quickly. For example, popular Linear Tape-Open drives can reach, as of 2010, continuous data transfer rates of up to 140 MB/s, which is comparable to hard disk drives.

Contents

Design

Tape drives can range in capacity from a few megabytes to hundreds of gigabytes of uncompressed data. In marketing materials, tape storage is usually referred to with the assumption of 2:1 compression ratio, so a tape drive might be known as 80/160, meaning that the true storage capacity is 80 whilst the compressed storage capacity can be approximately 160 in many situations. IBM and Sony have also used higher compression ratios in their marketing materials. The real-world, observed compression ratio always depends on what type of data is being compressed. The true storage capacity is also known as the native capacity or the raw capacity.

Tape drives can be connected to a computer with SCSI (most common), Fibre Channel, SATA, USB, FireWire, FICON, or other[1] interfaces. Tape drives can be found inside autoloaders and tape libraries which assist in loading, unloading and storing multiple tapes to further increase archive capacity.

Some older tape drives were designed as inexpensive alternatives to 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.

Problems

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.

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