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Red Hat Enterprise Linux 5/6 (RHEL 5/6 ) - Different I/O Scheduler Used in RHEL

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What are the different I/O scheduler used in RHEL?

Details

Input/output (I/O) scheduling is a term used to describe the method computer operating systems decide the order that block I/O operations will be submitted to storage volumes.

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.

There are four custom configured schedulers from which to choose. Each offer a different combination of optimizations.

The Completely Fair Queuing (CFQ) scheduler is the default algorithm in Red Hat Enterprise Linux 6.

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 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.

Reference:

Click here to access the technical article Non-HPE site at: http://en.wikipedia.org/wiki/I/O_scheduling.

Click here to access the technical article Non-HPE site at: http://www.redhat.com/magazine/008jun05/features/schedulers/.

NOTE: The above-mentioned URL will take you to a non-HP Web site. HP does not control and is not responsible for information outside of the HP Web site.

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