A disk operation request specifies several pieces of information:
- Is this input or output ?
- Disk address for the transfer.
- Memory address for the transfer.
- Number of bytes to be transferred.
When a request is initiated by a process, if the disk is available, the request can be serviced immediately.
If the disk is busy, a request queue can form.
Upon completion of a disk request, the operating system can choose which pending request to service next.
We discuss several disk scheduling algorithms that determine how to choose the next request to service.