Some Other Scheduling Issues

Where is the disk scheduling performed?

There are many other issues we do not discuss in the brief description of basic disk operation, scheduling, and related topics presented in the previous lesson. One such issue is this: where is disk scheduling performed on modern systems? In older systems, the operating system did all the scheduling; after looking through the set of pending requests, the OS would pick the best one, and issue it to the disk. When that request is completed, the next one would be chosen, and so forth. Disks were simpler then, and so was life.

In modern systems, disks can accommodate multiple outstanding requests, and have sophisticated internal schedulers themselves (which can implement SPTF accurately; inside the disk controller, all relevant details are available, including exact head position). Thus, the OS scheduler usually picks what it thinks the best few requests are and issues them all to disk. The disk then uses its internal knowledge of head position and detailed track layout information to service said requests in the best possible (SPTF) order.