Tape drive
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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. Image:Coloradobrandtapedrive.jpg
An external QIC tape drive.
Tape drives can be connected to a computer with SCSI (most common), Fibre Channel, FICON, ESCON, parallel port, IDE, USB, FireWire or other interfaces. Tape drives can range in capacity from a few megabytes to upwards of 800 GB. Tape drive 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 standard storage capacity is 80 whilst the compressed storage capacity can be up to 160. The raw storage capacity is known as the native capacity. Tape drives can be found inside autoloaders and tape libraries which assist in loading, unloading and storing multiple tapes to further increase archive capacity. In the 1980s some forms of tape drives were used as inexpensive alternatives to disk drives, examples include the ZX Microdrive and Rotronics Wafadrive. Shoe-shining effectThe 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 deccelerate 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; commonly employed mechanisms used to alleviate the problem were vacuum columns. 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
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