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Jan's Computer Basics:

Storage: Magnetic Tape

Magnetic tape uses a method similar to that of VCR tape for storing data.

The speed of access can be quite slow, however, when the tape is long and what you want is not near the start. So this method is used primarily for major backups of large amounts of data.

Businesses especially might do a backup of the day's transactions every day and a backup of the whole system once a week or so. Keeping sets of backups like this minimizes the amount of data loss when the computer system goes down.

Types of Tape

Each different tape storage system has its own requirements as to the size, the container type, and the magnetic characteristics of the tape. Older systems designed for networks use reel-to-reel tapes. Newer systems use cassettes. Some of these are even smaller than an audio cassette but hold more data than the huge reels. Even if they look alike, the magnetic characteristics of tapes can vary. It is important to use the tape that is right for the system.

tape reel tape cassette
Tape Reel Tape Cassette

Tape Formats

Just as hard disks have several different formats, so do magnetic tapes. The format method will determine the following characteristics.

Characteristic Looks like:
Density

Higher density means more data on shorter tape
Measured as bpi = bits per inch

Densities ranged up to 6250 bpi in the old days but now (2008) density can be much higher, such as 124,000 bpi.		 tape density  tape denser density
Blocks

The tape is divided into logical blocks, as a hard disk is divided into tracks and sectors. One file could take up many logical blocks, but must take up at least one whole block. So smaller blocks would result in less wasted space, leaving more room for data.

tape with logical blocks
Gap

Two kinds of blank spots, called gaps, are set on the tape.
   Interblock gap which separates logical blocks.
   Interrecord gap which is wider and separates records.

Notice the two size lines cutting across the tape in the picture above. Smaller gaps would allow more data to be stored on the same size tape.