Hard discs (both disc pack and Winchester) in fact consist of a number of disks mounted on a common spindle.

Data is stored in concentric rings called tracks. Normally a particular item is found on a disc by moving the read/write head in to the appropriate track and then waiting for the data required to come round and be read. In some large systems, where access time is critical, fixed heads are used.

There is one head per track so that the need to move the head to the correct track is eliminated. This improves access time but the cost is increased.

Data can be stored on any magnetic disc to allow either sequential or random access. If random access is used then the records must be fixed length (every record taking up the same amount of storage space) so that the position of a particular record on the disc can be calculated. Fixed length records are normally achieved by having fixed field lengths, padding out fields that are not long enough with space characters.

In a standard PC within the school the Hard Disk will be between 40 and 80 GigaBytes in size. The servers where student data is stored are obviously substantially larger. A typical Hard disk used in the school will last for 2 to 4 years. Aging depends on the degree of use but also the temperature - the cooler the better!

© LEV