Storage : Magnetic Tapes

Magnetic tape comes in two different forms :

The storage capacities of tape cartridges are now so great that reels of tape are not used in new computer systems.

A computer tape is divided up into many blocks. A block on a tape is like a sector on a disk. To read data from a particular block on the tape all of the blocks that come before it must be read first. This means that finding a particular file/record on a tape can take much longer than finding a particular file/record on a disk. However once the file or record has been located it can be read more quickly from a tape than in could be from a hard disk. Having to read through all of the files/records that are stored before the file/record you want to access is known as serial access.

Often files or records are stored on a tape in a particular order (e.g. sorted alphabetically by a key field). If this is the case then the tape is described as having sequential access. Because locating data on a tape takes a long time, magnetic tapes are not used as general purpose storage devices. They are only useful for a few applications. The two main applications tape is used for are :

Applications for Magnetic Tapes
Backup	Often a tape streamer is used to make copies of data stored on a hard disk in case the data becomes corrupted. If this happens then the correct data can be restored from the tape. A backup copy of the contents of the hard disk could be made once every week. Tapes are more suitable for making backups than floppy disks or CD-ROMs. This is because the entire contents of a hard disk can be written onto one tape, producing the backup will be much quicker and tapes are cheaper to buy. It does not matter that tapes are serial access because when a backup is made or restored every file on the tape has to be accessed.
Batch Processing	In a batch processing system when processing occurs the whole master file and the whole transaction file are read to produce a new master file. Because the master file and transaction file are both sorted into the same order the data can be read sequentially from each file. A batch processing system does not have to move backwards and forwards through a tape looking for records. The system is designed so that the next record to be read is stored immediately after the current record on the tape. Therefore using a serial access medium instead of a direct access medium will not slow down processing. Tapes are preferred to disks in batch processing systems due to their relative low costs and fast read/write speeds.

GCSE ICT Companion 04 - (C) P Meakin 2004