IOSchedulers

IO schedulers, or block I/O schedulers, are components of the operating system kernel that determine the order in which I/O requests to storage devices are issued. They aim to optimize throughput, latency, and fairness by reordering pending read and write requests within per-device queues. The scheduler's decisions affect how quickly interactive processes respond and how efficiently storage devices are utilized, especially under concurrent workloads.

A typical scheduler maintains a queue of outstanding requests and selects the next request based on its algorithm. Algorithms range from simple to complex. Elevator-based approaches (SCAN, LOOK, or their variants) reorder requests to move a disk head in one direction, reducing seek time and improving throughput on hard drives. Deadline scheduling assigns deadlines to requests and favors those nearing their deadlines, reducing latency for time-sensitive tasks. Completely Fair Queuing (CFQ) attempts to share disk bandwidth fairly among processes or cgroups, balancing responsiveness and throughput. Budget Fair Queueing (BFQ) refines fairness with per-client budgets to control service discipline. No-Op scheduling minimizes reordering, delegating most work to the device, and is often used for solid-state drives where reorder latency benefits are minimal. Multi-queue variants, such as mq-deadline or kyber, extend scheduling to multi-queue devices and modern storage stacks.

Most operating systems offer a choice of IO schedulers and allow switching between them to suit workload and hardware. Tuning can involve selecting an appropriate scheduler, enabling or disabling reordering for SSDs, and adjusting per-device parameters related to queue depth and fairness.