I/O scheduling

For process scheduling, see scheduling (computing).

For process management, see process management (computing).

I/O Scheduling is the term used to describe the method computer operating systems decide the order that block I/O operations will be submitted to the disk subsystem. I/O Scheduling is sometimes called 'disk scheduling'.

Purpose

I/O schedulers can have many purposes depending on the goal of the I/O scheduler, some common goals are:

• To minimize time wasted by hard disk seeks.
• To prioritize a certain processes' I/O requests.
• To give a share of the disk bandwidth to each running process.
• To guarantee that certain requests will be issued before a particular deadline.

Implementation

I/O Scheduling usually has to work with hard disks which share the property that there is long access time for requests which are far away from the current position of the disk head (this operation is called a seek). To minimise the affect this has on system performance, most I/O schedulers implement a variant of the elevator algorithm which re-orders the incoming randomly ordered requests into the order in which they will be found on the disk.

Common disk scheduling disciplines

• Random Scheduling (RSS)
• First In, First Out (FIFO), also known as First Come First Served (FCFS)
• Last In, First Out (LIFO)
• Shortest seek first, also known as Shortest Seek / Service Time First (SSTF)
• Elevator algorithm, also known as SCAN (including its variants, C-SCAN, LOOK, and C-LOOK)
• N-Step-SCAN SCAN of N records at a time
• FSCAN, N-Step-SCAN where N equals queue size at start of the SCAN cycle.
• Completely Fair Queuing (Linux)
• Anticipatory scheduling

See also

• Tagged Command Queuing (TCQ)
• Native Command Queuing (NCQ)

References

• Love, R. (2005). Linux Kernel Development, Novell Press. ISBN 0-672-32720-1