Magnetic Tapes/Drives
One
of the first storage media used with mainframe computers was the magnetic
tape, a magnetically coated ribbon of plastic capable of storing large
amounts of data and information. Magnetic tape storage requires sequential
access, which refers to reading or writing data consecutively. Like
a music tape, you must forward or rewind the tape to a specific piece of
data. Floppy disks, hard disks, and compact discs all use direct
access, or random access, which means you can locate a particular data
item of a file immediately, without having to move consecutively through
items stored in front of the desired data item of a file. Because sequential
access is much slower than direct access, tapes are no longer used as a
primary methods of storage.
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Similar
to a tape recorder, a tape drive is used to read from and write data and
information to tapes. Although older computers used reel-to-reel
tape, today's tape drives use tape cartridges which are small, rectangular,
plastic housings for a tape. Tape cartridges containing one-quarter-inch
wide tape are slightly larger than audiocassette tapes and frequently are
used for personal computer backup. Some personal computes have permanently
mounted tape drives, while others have external units. On larger computers,
tape cartridges are mounted in a separate cabinet called a tape library.
The library integrates many tape drives with sophisticated robotics that
minimize human error, the most prominent cause of data loss. These
tapes and libraries have a large range of memory ranging from the storage
of a fraction of a PC's memory to a large corporation’s entire database.
As far as individual tapes are concerned, a quarter-inch cartridge ranges
from 40 MB to 5 GB, a digital audio tape from 2 GB to 24 GB, and a digital
linear tapes from 20 MB to 40 MB. Unfortunately, the tapes that hold
the most information tend to have the most expensive drives with the slowest
transfer rates.
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Now,
a tape drive is most often used to copy or backup a hard disk. Should
the hard disk data be destroyed or corrupted, it could then be quickly
and accurately restored from the backup tape. One of the biggest
advantages of storing information on tapes is the price differential between
5 cents per megabyte on a hard disk, compared to just .2 cents on a tape.
On the negative side, data restoration becomes a time consuming and expensive
ordeal when a tape backup system is in place.
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1. Low Cost
Magnetic
tapes are the least expensive medium for data storage. On a per gigabyte
basis, the tape costs from $1 to $5. In comparison, hard disk drives cost
approximately $65 to $110 per gigabyte. Though less expensive than
hard disks, technologies such as CD-ROM, DVD and magneto-optical (MO) are
still more expensive than tapes, ranging anywhere from $3 to $25 per gigabyte.
In addition to being more expensive than tapes, the relatively low capacity
(per cartridge) of these devices prohibits them from being an efficient
medium for backup.
2. High Capacity
The
relatively high per cartridge capacity of tapes is very comparable to the
increasing capacity of hard disks. The majority of disk drives being sold
today are in the 9 GB to 18 GB range. Several tape technologies offer capacity
in the range of 12 GB to 20 GB per cartridge, which is a good match for
backing up those popular disk drive sizes. As technology grows, disk
drive and tape capacities grow as well. In comparison, other removable
technologies such as CD-ROM, DVD and MO range in per unit capacity from
650 MB to 5 GB.
3. Removability
Removability
is an important criteria for any backup technology. The backup copy
should be physically separated from the primary data source (server).
This can only be accomplished with a removable storage technology (or by
employing the complex and expensive process of remote mirroring).
Since tapes are removable, the administrator can store the duplicate data
in a secure location such as a locked closet or cabinet or at a remote
site.
4. Reliability
Another
advantage of tapes is that they have been used as a backup medium for over
two decades. Tapes have been continually refined to meet the ever-increasing
demands of the computer industry and have proven to be a reliable, cost-efficient
medium for high-capacity data storage.
In
addition to backup, tapes are also ideal for other applications including:
- Archiving
- Near-line data storage
- Off-line data storage within hierarchical storage management architectures (HSM)
- Data collection and distribution
- Software distribution
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Last Edited On May 14, 2000