LOOK algorithm

LOOK is a disk scheduling algorithm used to determine the order in which new disk read and write requests are processed.

Description

LOOK is similar to SCAN in that the heads sweep across the disk surface in both directions performing reads and writes. However, unlike SCAN, which visits the innermost and outermost cylinders each sweep, LOOK will change directions when it has reached the last request in the current direction.

Variants

One variant of LOOK is C-LOOK, which fulfills requests in one direction only. That is, C-LOOK starts at the innermost cylinder requested and moves outward fulfilling requests until it reaches the last request. Then it moves directly back to the innermost request and starts fulfilling requests moving outward again.

Performance

LOOK has slightly better average seek times than SCAN. C-LOOK has a slightly lower variance in seek time than LOOK since the worst case seek time is nearly cut in half.

See also

Other variations include:

• SCAN - Elevator algorithm
• FSCAN
• N-Step-SCAN