Cscan

C-SCAN, short for Circular SCAN, is a disk scheduling algorithm used by operating systems to determine the order in which disk I/O requests are serviced. It is a variant of the SCAN algorithm designed to provide more uniform wait times across requests.

How it works: The disk is modeled with cylinders numbered from 0 to N−1. Pending I/O requests are kept in a queue. The read/write head moves in a single direction (typically toward higher-numbered cylinders), servicing all requests encountered along the way. When the head reaches the last cylinder, it jumps to the first cylinder (0) and continues moving in the same direction, again servicing requests as it progresses. The wrap-around is treated as a full traversal of the disk, and the head does not service requests during the return trip to the start.

Advantages and characteristics: C-SCAN provides a more uniform waiting time than SCAN, since every request waits at most for a full rotation to be serviced in the worst case. This makes response times more predictable for workloads with many requests spread across the disk. It is simple to implement by maintaining a sorted structure of requests and moving the head in one direction, then wrapping around.

Disadvantages: The wrap-around can incur a substantial seek distance for workloads concentrated near the middle or end of the disk, potentially increasing total head movement compared to other policies.

Variants: Some implementations treat the wrap distance differently, or count the wrap as zero, but the core idea remains a circular service pattern with unidirectional sweeps between wrap-arounds. C-SCAN is commonly contrasted with SCAN and SSTF in discussions of disk scheduling.