Generally, the I/O subsystem does not operate in a true FIFO manner. It processes queued read/write requests depending on the selected scheduler algorithms. These scheduler algorithms are called elevators. Elevators were introduced in the 2.6 kernel.

Scheduler algorithms are sometimes called “elevators” because they operate in the same manner that real-life building elevators do. The algorithms used to operate real-life building elevators make sure that it services requests per floor efficiently. To be efficient, the elevator does not travel to each floor depending on which one issued a request to go up or down first. Instead, it moves in one direction at a time, taking as many requests as it can until it reaches the highest or lowest floor, then does the same in the opposite direction.

Simply put, these algorithms schedule disk I/O requests according to which logical block address on disk they are targeted to. This is because the most efficient way to access the disk is to keep the access pattern as sequential (i.e. moving in one direction) as possible. Sequential, in this case, means “by increasing logical block address number”.

As such, a disk I/O request targeted for disk block 100 will normally be scheduled before a disk I/O request targeted for disk block 200. This is typically the case, even if the disk I/O request for disk block 200 was issued first.

However, the scheduler/elevator also takes into consideration the need for ALL disk I/O requests (except for read-ahead requests) to be processed at some point. This means that the I/O subsystem will not keep putting off a disk I/O request for disk block 200 simply because other requests with lower disk address numbers keep appearing. The conditions which dictate the latency of unconditional disk I/O scheduling is also set by the selected elevator (along with any specified request queue parameters).

There are several types of schedulers:

These scheduler types are discussed individually in the following sections.