The Linux kernel, the core of the operating system, is responsible for controlling disk access by using kernel IO scheduling.
This article explains how-to change the IO scheduler without recompiling the kernel and without restart.
Check the currently used IO scheduler
noop anticipatory [deadline] cfq
For example the scheduler deadline delivers best performance on hardware raid and SAN environments. While noop delivers better performance for SSDs.
Switching IO Schedulers on runtime
Set the scheduler for /dev/sda to Deadline:
echo deadline > /sys/block/sda/queue/scheduler
Set the scheduler for /dev/sda to CFQ:
echo cfq > /sys/block/sda/queue/scheduler
Set IO Schedulers permanently
In order to choose a new default scheduler you need to add the following into your /etc/default/grub:
After you change /etc/default/grub you need to run update-grub to apply changes: