Tape drive

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DDS tape drive. Above, from left to right: DDS-4 tape (20 GB), 112m Data8 tape (2.5 GB), QIC DC-6250 tape (250 MB), and a 3.5" floppy disk (1.44 MB)

A tape drive is a data storage device that reads and writes data stored on a magnetic tape. It is typically used for off-line, 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, modern LTO drives can reach continuous data transfer rates of up to 80 MB/s, which is as fast as most 10,000 RPM hard disks.

[edit] Design

An external QIC tape drive.

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.

[edit] 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.

In early tape drives, the situation of non-continuous data transfers was normal and unavoidable - weak computer processors and memory were rarely able 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 drive designs of the 1980s introduced the use of an internal data buffer to somewhat reduce start-stop situations. These drives are colloquially referred to as 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 the tape speed increased, the start-stop operation was no longer possible, and the drives started to suffer from shoe-shining (sequence of stop, rewind, start).

Most recently, drives no longer operate at single fixed linear speed, but have a few speed levels. Internally, they implement algorithms that dynamically match the tape speed level to computer's data rate. Example speed levels could be 50 percent, 75 percent and 100 percent of full speed. Still, a computer that streams data constantly below the lowest speed level (e.g. at 49 percent) will undoubtedly cause shoe-shining.

When shoe-shining occurs, it significantly affects the attainable data rate, as well drive and tape life.

[edit] Media

Magnetic tape is commonly housed in a casing such as plastic known as a cassette or a 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 plastic outer shell permits ease of handling of the fragile tape, making it far more convenient and robust than having loose or exposed tape.

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