DISK BLOCK COMPONENTS

THE TIME TO ACCESS A DISK BLOCK HAS FOLLOWING COMPONENTS

1. Seek time : Seek time is defined as the time taken to move the disk heads to the track on which a desired block is located.  
2. Rotational delay : Rotational delay is defined as the waiting time for the desired block to rotate under the disk head; it is the time required for half a rotation on average and is usually less than seek time.
3. Transfer Time : Transfer time is defined as the time to actually read or write the data in the block once the head is positioned. that is, the time for the disk to rotate over the block. 

THE PERFORMANCE OF DISK STRUCTURE DEPENDS UPON THE FOLLOWING THREE PRINCIPLES :

1. Data must be in memory for the DBMS to operate on it.
2. The unit for data transfer between disk and main memory is block; if a single item on a block is needed. the entire block is transferred. Reading or writing a disk block is called an I/O (input/output) operation.
3. The time to read for write a block varies, depending on the location of the data : 

             access time = seek time + rotational delay + transfer time. 

These observations imply that the time taken for database operations is affected significantly by how data is stored on disks.