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A tape drive is a data storage device that reads and writes data on a magnetic
tape. Magnetic tape data storage 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 to any position on the
disk in a few milliseconds, but a tape drive must physically wind tape between
reels to read any one particular piece of data. As a result, tape drives have
very slow average seek times. For sequential access once the tape is positioned,
however, tape drives can stream data very fast. For example, as of 2010 Linear
Tape-Open (LTO) supported continuous data transfer rates of up to 140 MB/s,
comparable to hard disk drives. Contents
Design
Tape drives can range in capacity from a few megabytes to hundreds of gigabytes
of uncompressed data.
As some data can be compressed to a smaller size than the files on hard disc, it
has become commonplace when marketing tape drives to state the capacity with the
assumption of a 2:1 compression ratio; thus a tape with a capacity of 80 GB
would be sold as "80/160". The true storage capacity is also known as the native
capacity or the raw capacity. IBM and Sony have also used higher compression
ratios in their marketing materials. The compression ratio actually achievable
depends on the data being compressed. Some data has little redundancy; large
video files, for example, already use compression technology and cannot be
compressed further. A sparse database, on the other hand, may allow compression
ratios better than 10:1.
Tape drives can be connected to a computer with SCSI (most common), Fibre
Channel, SATA, USB, FireWire, FICON, or other interfaces. Tape drives are
used with autoloaders and tape libraries which automatically load, unload, and
store multiple tapes, increasing the volume of data which can be stored without
manual intervention.
Some older tape drives - such as the DECtape, the ZX Microdrive and the
Rotronics Wafadrive - were designed as inexpensive alternatives to disk drives
which were at the time very expensive. However, modern tape drives that used
advanced techniques like multilevel forward error correction, shingling, and
linear serpentine layout for writing data to tape, along with lower disk drive
prices, have made such alternatives obsolete.
Reliability
Storage Magazine and Gartner reported that 34% of surveyed companies never test
a restore from tape.
Technical problems
A disadvantageous effect termed "shoe-shining" occurs during read/write 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. In
this situation, the modern fast-running tape drive is unable to stop the tape
instantly. Instead, 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. If the condition repeats, the resulting
back-and-forth tape motion resembles that of shining shoes with a cloth. Shoe-shining
decreases the attainable data transfer rate, drive and tape life, and tape
capacity.
In early tape drives, non-continuous data transfer was normal and unavoidable.
Computer processing power and amounts of available memory were usually
insufficient 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.
Some modern designs are still developed to operate in a non-linear fashion. IBM's
3xxx formats are designed to keep the tape moving irrespective of the data
buffer - segments are written when data is available, but gaps are written when
buffers run empty. When the drive detects an idle period, it re-reads the
fragmented segments into a buffer and writes them back over the fragmented
sections - a 'virtual backhitch'.
Later, most tape drives of the 1980s introduced the use of an internal data
buffer to somewhat reduce start-stop situations. These drives are often referred
to as tape 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 faster tape
drives became available, despite being buffered the drives started to suffer
from the shoe-shining sequence of stop, rewind, start.
Most recently, drives no longer operate at a single fixed linear speed, but have
several speeds. Internally, they implement algorithms that dynamically match the
tape speed level to the computer's data rate. Example speed levels could be 50
percent, 75 percent and 100 percent of full speed. A computer that streams data
slower than the lowest speed level (e.g. at 49 percent) will still cause shoe-shining.
Media
Magnetic tape is commonly housed in a casing known as a cassette or 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
outer shell, made of plastic, sometimes with metal plates and parts, permits
ease of handling of the fragile tape, making it far more convenient and robust
than having spools of exposed tape. Simple analog Compact Cassette audio tape
recorders were commonly used for data storage and distribution on home computers
at a time when floppy disk drives were very expensive. The Commodore Datasette
was a dedicated data version using the same media.
Market Trends
In 2007, Gartner analyst Dave Russell predicted that recovery will move from
tape to online disk-based storage by 2011, causing a major shift in the backup
market.
Future Capacity
Tape drives have yet to reach their maximum capacity.
In 2011, Fujifilm and IBM announced that they had managed to record 29.5 billion
bits per square inch with magnetic tape media developed using the BaFe particles
and nanotechnologies allowing for an uncompressed tape drive of 35TB.
The technology is not expected to be commercially available for at least another
decade.