Selectable I/O schedulers for 2.6

[Posted September 24, 2003 by corbet]

The 2.5 development series saw the creation of a few different I/O schedulers ("elevators") for the block I/O subsystem. I/O schedulers attempt to perform requested block I/O operations in an order that maximizes performance. Given that different people (and applications) measure performance differently, it is not surprising that more than one I/O scheduler exists. So, for example, the "deadline" scheduler attempts to minimize seeks while ensuring that no request waits for more than a certain period of time. The anticipatory scheduler pauses after completing read operations on the assumption that another nearby read will show up quickly. The CFQ ("completely fair queueing") scheduler tries to divide up the available I/O bandwidth equally among processes. And there is a "noop" scheduler for devices (such as memory-based devices) which do not benefit from I/O scheduling logic at all.

What has been lacking is any sort of way for a system administrator to choose between these schedulers. A system I/O scheduler can be designated with the elevator= boot parameter, but that choice applies to all drives on the system, and it cannot be changed. This restriction makes experimenting with the various schedulers difficult; in the real world, it may also be appropriate to use different schedulers for different drives.

So Nick Piggin has released a patch which makes I/O schedulers selectable at run time. With the patch, a new io_scheduler sysfs attribute appears under /sys/block/<device>/queue; changing a scheduler is simply a matter of writing the name of the new scheduler into that attribute. So, for example, to go to CFQ on the first SCSI drive:

    echo cfq >/sys/block/sda/queue/io_scheduler

Changing schedulers requires pausing and emptying the I/O queue, so it might not be advisable in the middle of writing a CD or controlling a nuclear power plant shutdown. But it certainly can be a useful thing to do at system initialization time, or while experimenting with scheduler performance under a certain kind of load.