tape drive

A tape drive, which is also known as a streamer, is a data storage device that reads and writes data stored on a magnetic tape. It is typically used for archival storage of data stored on hard drives. Tape media generally has a favorable unit cost and long archival stability.

Instead of allowing random-access to data as hard disk drives do, tape drives only allow for sequential-access of data. A hard disk drive can move its read/write heads to any random part of the disk platters 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, tapes drives can stream data to 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.

Tape drives can range in capacity from a few megabytes to hundreds of gigabytes, uncompressed. 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, FICON, ESCON, parallel port, IDE, SATA, USB, FireWire or other 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.

Shoe-shining effect

The shoe-shining effect occurs during writing or reading data to tape, when the transfer rate of the data falls below the minimum threshold at which the tape drive heads were designed to transfer data to a running tape. When this occurs, the drive must decelerate the tape, stop it, rewind back a little, accelerate again to a proper speed and continue writing from the same position.

In early drives, such start-stop work was often unavoidable. Vacuum columns were commonly employed to minimize the problem. The loops of tape hanging in the vacuum columns on either side of the tape heads had far less interia than the two reels that the rest of the tape was stored on.

Later, most tape drive designs of the 1980s introduced the internal data buffer to somewhat reduce start-stop situations. The tape was stopped only when the buffer contained no data to be written (buffer underflow), or when it was full of data during reading (buffer overflow).

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%, 75% and 100% of full speed. Still, a computer that streams data constantly below the lowest speed level (e.g. at 49%) will undoubtedly cause shoe-shining.

When shoe-shining occurs, it significantly affects the attainable data rate. It is most important in backup process to modern fast drives. Furthermore, shoe-shining places undue stress on the drive mechanism and the tape medium itself, increasing hardware failure rate.

Advancements in the history of tape drives

Year Manufacturer Model Advancements
1951 Remington Rand UNISERVO First computer tape drive
1952 IBM 726 Use of plastic tape (cellulose acetate)
1958 IBM 729 Separate read/write heads providing transparent read-after-write verification
1972 3M QIC-11 Tape cassette (with two reels)
1974 IBM 3850 Tape cartridge (with single reel) First tape library with robotic access
1980 Cipher (F880?) RAM buffer to mask start-stop delays
1984 IBM 3480 Internal takeup reel with automatic tape takeup mechanism. Thin-film magnetoresistive (MR) head.
1984 DEC TK50 Linear serpentine recording
1986 IBM 3480 Hardware data compression (IDRC algorithm)
1987 Exabyte/Sony EXB-8200 First helical digital tape drive. Elimination of the capstan and pinch-roller system.
1993 DEC Tx87 Tape directory (database with first tapemark nr on each serpentine pass).
1995 IBM 3570 Head assembly that follows pre-recorded tape servo tracks (Time Based Servoing or TBS)
Tape on unload rewound to the midpoint - halving access time (requires two-reel cassette, resulting in lesser capacity)
1996 HP DDS3 Partial Response Maximum Likelihood (PRML) reading method - no fixed thresholds
1997 IBM VTS Virtual tape - disk cache that emulates tape drive
1999 Exabyte Mammoth-2 The small cloth-covered wheel cleaning tape heads.
Inactive burnishing heads to prep the tape and deflect any debris or excess lubricant.
Section of cleaning material at the beginning of each data tape.
2000 Quantum Super DLT optical servo allows more precise positioning of the heads relative to the tape
2003 IBM 3592 Virtual backhitch
2003 Sony SAIT-1 Single-reel cartridge for helical recording
2006 StorageTek T10000 Multiple head assemblies and servos per drive
2007 IBM 3592 Encryption capability integrated into the drive
2008 IBM TS1130 GMR heads in a linear tape drive

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