i/o Scheduler for Disk in Linux :How to ??

Why does kernel need IO scheduler?

Without an I/O scheduler, the kernel would basically just issue each request to disk in the order that it received them. This could result in massive HardDisk thrashing: if one process was reading from one part of the disk, and one writing to another,
the heads would have to seek back and forth across the disk for every operation. The scheduler’s main goal is to optimise disk access times.


How to view Current Disk scheduler ?

Assuming that we have a disk name /dev/sda, type :

# cat /sys/block/{DEVICE-NAME}/queue/scheduler
# cat /sys/block/sda/queue/scheduler



How to set I/O Scheduler For A Hard Disk ?

To set a specific scheduler, simply type the command as follows:

# echo {SCHEDULER-NAME} > /sys/block/{DEVICE-NAME}/queue/scheduler
For example, set noop scheduler, enter:
# echo noop > /sys/block/hda/queue/scheduler

OR

Edit /boot/grub/grub.conf and enter in kernel line "elevator=noop" or any other scheduler available.

There are currently 4 available IO schedulers :

* No-op Scheduler
* Anticipatory IO Scheduler (AS)
* Deadline Scheduler
* Complete Fair Queueing Scheduler (CFQ)


Changing Scheduler:

The most reliable way to change schedulers is to set the kernel option “elevator” at boot time. You can set it to one of “as”, “cfq”, “deadline” or “noop”, to set the appropriate scheduler.

elevator=cfq