Storage Of Database On Hard Disks
At this point, it is worthwhile to note the difference among the terms file Organisation and the access method. A file organisation shows to the organisation of the data of a file into blocks, records and access structures; this contains the way records and blocks are placed on the storage medium and interconnected. An access method, on the other hand, is the mode how the data can be retrieved based on the file Organisation.
Mainly the databases are stored persistently on magnetic disks for the reasons given below:
- The databases being very big may not set completely in the main memory.
- Storing the data permanently using the non-volatile storage and give access to the users with the help of front end applications.
- Primary storage is considered to be very costly and in order to cut short the cost of the storage per unit of data to substantially less.
Every hard drive is usually composed of a set of disk platters. Every disk platter has a layer of magnetic material deposited on its surface. The whole disk can have a large amount of data, which is organised into minor packages known as BLOCKS (or pages). On most computers, one block is equal to 1 KB of data (= 1024 Bytes).
A block is the least unit of data transfer among the processor and the hard disk of the computer. Every block thus has a fixed, address, assigned. Typically, the computer processor will give in a read/write request, which contains the address of the block, and the address of RAM in the computer memory area known as a buffer (or cache) where the data must be stored / taken from. The processor then reads and changes the buffer data as needed, and, if needs, writes the block back to the disk. Let us observe how the tables of the database are stored on the hard disk.