Database Reference
In-Depth Information
do not require power or a conditioned computer room environment to retain
data or be prepared for use. Tape libraries requiring very little power continue
to keep tape access times reasonable for sites with requirements to keep vast
amounts of data accessible to users. Archival life of tape is very good, with
most tapes capable of retaining data anywhere from 15-30 years.
1.4.1 Fundamentals
Tape media has been made for about a decade and is composed of a durable
substrate, mylar in most media, with single- or dual-layer magnetic alloys.
Tape cartridges have tracks with data blocks or sections written to the tracks.
Depending on the tape drive or application using the tape, file marks are
written to the tape to serve as markers for moving to different points or
data blocks on the tape. The primary down side of tape is its sequential
access limitations. Tape does not lend itself to random access because tape
is a continuous medium that must be moved forward and backward until the
desired position is reached. The further down the tape the desired data resides,
the longer it takes to reach the data.
Cartridges can be single or multireel. Single-reel tapes are fed through the
drive with rollers where the tape passes over the recording head of the drive.
Dual-reel tapes are loaded into the drive but keep the tape internal to the
cartridge with the recording head of the tape drive positioning itself over
the tape between the two reels. Tape drive speed and tape recording head
capabilities determine the tape's data transfer rate. Tape can be damaged
given the high velocity and mechanical nature of reading and writing data.
However, data loss is normally limited to a small part of a single tape cartridge.
There are several types of tape technology in use in the high-performance
computing industry. Tape drives utilize SCSI and Fiber Channel protocols.
New tape drive models usually demand new tape media formats. Commodity
tape drives typically don't plan for media reusability with multiple versions
of drives, whereas enterprise tape drives plan for media reuse and backward
compatibility. Most tape drives are capable of and automatically compress
data being written to tape. The largest tape formats allow nearly 1 TB of
data uncompressed to fit on a single tape. The cost of storing data on tape
is typically an order of magnitude less than the cost of storing the same data
on disk, irrespective of upkeep, maintenance, or power costs.
Access times for tape depend on the tape model used, but for the high-
est performing tape libraries, they typically range from 30 seconds to several
minutes to mount the tape and deliver the first byte of data. The highest
performing tape drives are capable of transferring uncompressed data at 180
MB/s and achieve about 380 MB/s with compression. Access time is highly de-
pendent on the tape drive's seek capabilities. Tape is fundamentally sequential
media, and tape marks are essential to finding the exact beginning and end of
user data. Tape marks are a unique pattern written to tape by the drive when
requested by the application in order to separate user data on the tape. This
