Real-time disk scheduling in a mixed-media file system

This paper presents our real-time disk scheduler called the ΔL scheduler, which optimizes unscheduled best-effort disk requests by giving priority to best-effort disk requests while meeting real-time request deadlines. Our scheduler tries to execute real-time disk requests as much as possible in the background. Only when real-time request deadlines are endangered, our scheduler gives priority to real-time disk requests. The ΔL disk scheduler is part of our mixed-media file system called Clockwise. An essential part of our work are extensive and detailed raw disk performance measurements. These raw performance measurements are used by the ΔL disk scheduler for its real-time schedulability analysis and to decide whether scheduling a best-effort request before a real-time request violates real-time constraints. Further, the raw performance measurements are used by a Clockwise off-line simulator where a number of different disk schedulers are compared. We compare the ΔL scheduler with a prioritizing Latest Start Time (LST) scheduler and non-prioritizing EDF scheduler. The ΔL scheduler is comparable to LST in achieving low latencies for best-effort request under light to moderate real-time loads and better in achieving low latencies for best-effort requests for extreme real-time loads. The simulator is calibrated to an actual Clockwise