Magnetic tape

A strip of iron oxide coated plastic film, which is a low cost, high storage capacity, sequential media for storing data. A sequential Access Device, primarily used as a cheap way to store "off-line" data.(Such as Archive data - no longer required on-line) Since sequential access requires the whole tape to be "restored" data can be compressed onto the tape. Due to its low cost and large storage capacity, it is ideal as a backup media. In some cases CD's are replacing tape as backup media. Typical capacity(1998): Travan (3.2-96GB), DAT (2-96 GB), AIT (to 200GB)

• Magnetic tape is used to provide relatively cheap storage of data.
• Used for
  • Backup.
    • Able to store large amounts since data compression can occur.
    • In many instances being replaced with CD-ROM).
  • Archiving (Saving data no longer required on-line).
    • Can be a problem since tape formats are continually changing.

• A continuous strip of iron oxide coated plastic film
• Passed over a fixed read/write head and wound onto a takeup reel
• Takeup reel is inside a vacuum chamber, to cushion acceleration or deceleration forces from stretching the tape.
• Many metres of tape may have to be wound through to get to the start of a section of data, the access time is very much greater than for a disk, therefore tapes are not used for direct access devices.
• Tapes are often referred to as sequential access devices
• The data transfer rate is still quite high.
• Since it is a sequential access device the files are usually compressed as they are copied to the tape. This leads to considerable amounts of data "squeezing" onto the tape. A restore program is then required to read the data.

Note that a tape format is used in backup utilities used in Microsoft Windows, and floppy disks may be used instead of tapes. The backup utility compresses the files, so a Restore program is required to recover the files.

(1998, Aug) Seagate advertisement

• For each file on the tape, data about the file needs to be kept, such as the name of the file, its creation data and so on. This is called the "Header record".
• The tape also contains interblock gaps so that the tape may be read quickly with time to slow down the tape without snapping it.

• Manages data in chunks.

Example of a tape backup:

1. Copy FAT (with file selection data) into RAM buffer
2. Copy this to the tape controllers buffer
3. Copy data to tape
4. Copy file to RAM buffer
5. Copy this to the tape controllers buffer, while compressing file.
6. Copy data to tape

.. repeat 4-6 until backup complete.

( A common hardware buffer you will encounter is associated with the keyboard - if you "type" ahead of the screen, the computer remembers the keystrokes and implements them when the CPU catches up. This can create some interesting situations!)

• Low drive and media costs convince most users with high capacity hard drives to purchase a tape drive.
• Most will fit into the PC drive storage bays - if available.

[Rev: 24/8/98] 3/6/97 © 1997-98 V/2-Com (Verhaart), P O Box 8415, Havelock North, New Zealand.