The first issue in multiplexing multiple requests is doing so efficiently. Efficiently in this case means with returning data to the users as quickly as possible. Most of the latency in serving a disk request is in seek time, i.e., moving the disk arm over the proper track. Scheduling the requests to minimize seek time is often handled by the device driver. Scheduling user disk requests is called arm scheduling, because the problem is essentially scheduling accesses by the disk arm.
Some familiar algorithms appear, as well as some new ones.
This is easy to implement, but almost never used. Without optimizing at all, seek times can vary widely based on the applications making requests. Lone accesses to distant tracks suffer very long access times, or, in the worst case, never get served at all. A compromize between optimization and fairness is needed. The elevator algorithm tries to keep the disk arm moving one direction.