I/O scheduling controls how input/output operations will be submitted to storage.

The Completely Fair Queuing (CFQ) scheduler is the default algorthim in Red Hat Enterprise Linux and RHE family OS like Centos/Fedora and etc.. As the name implies, CFQ maintains a scalable per-process I/O queue and attempts to distribute the available I/O bandwidth equally among all I/O requests. CFQ is well suited for mid-to-large multi-processor systems and for systems which require balanced I/O performance over multiple LUNs and I/O controllers.

The Deadline elevator uses a deadline algorithm to minimize I/O latency for a given I/O request. The scheduler provides near real-time behavior and uses a round robin policy to attempt to be fair among multiple I/O requests and to avoid process starvation. Using five I/O queues, this scheduler will aggressively re-order requests to improve I/O performance.

The NOOP scheduler is a simple FIFO queue and uses the minimal amount of CPU/instructions per I/O to accomplish the basic merging and sorting functionality to complete the I/O. It assumes performance of the I/O has been or will be optimized at the block device (memory-disk) or with an intelligent HBA or externally attached controller.

The Anticipatory elevator introduces a controlled delay before dispatching the I/O to attempt to aggregate and/or re-order requests improving locality and reducing disk seek operations. This algorithm is intended to optimize systems with small or slow disk subsystems. One artifact of using the AS scheduler can be higher I/O latency.

cat /sys/block/sda/queue/scheduler

noop deadline [cfq]

cfq is default on Fedora 19, I mean my current OS 🙂

If you want to change scheduler: echo noop > /sys/block/sda/queue/scheduler

A little bit more:

• noop is often the best choice for memory-backed block devices (e.g. ramdisks) and other non-rotational media (flash) where trying to reschedule I/O is a waste of resources
• as (anticipatory) is conceptually similar to deadline, but with more heuristics that often improve performance (but sometimes can decrease it)
• deadline is a lightweight scheduler which tries to put a hard limit on latency
• cfq tries to maintain system-wide fairness of I/O bandwidth

The default was anticipatory for a long time, and it received a lot of tuning. cfq became the default some while ago, as its performance is reasonable and fairness is a good goal for multi-user systems (and even single-user desktops). For some scenarios — databases are often used as examples, as they tend to already have their own peculiar scheduling and access patterns, and are often the most important service (so who cares about fairness?) — anticipatory has a long history of being tunable for best performance on these workloads, and deadline very quickly passes all requests through to the underlying device.