I/O Scheduling is the term used to describe the method computer operating systems decide the order that block I/O operations will be submitted to the disk subsystem. I/O Scheduling is sometimes called 'disk scheduling'.
I/O schedulers can have many purposes depending on the goal of the I/O scheduler, some common goals are:
I/O Scheduling usually has to work with hard disks which share the property that there is long access time for requests which are far away from the current position of the disk head (this operation is called a seek). To minimise the effect this has on system performance, most I/O schedulers implement a variant of the elevator algorithm which re-orders the incoming randomly ordered requests into the order in which they will be found on the disk.