3. Data storage

Data is stored permanently in a mass storage device, usually a hard disk (see Magnetic and optical storage devices ). Data is stored on concentric circular tracks divided into several sectors. A block is made up of several contiguous sectors of the same track, and corresponds to the elementary quantity of information that can be read or written in a single operation. The size of a block, generally 4 kB, is set when the disk is formatted and is independent of the database's specificities.

The access time to any given block depends on the radial travel time of the read head to the track, the rotation of the disk until the first sector to be read is placed under the head, and the time taken to read the sectors. The performance of a hard disk is measured by two values: latency corresponds to the average time taken for the read head to move to any page,...