A preemptive priority-based disk I/O subsystem for the management of hard real-time disk traffic

We present the design and implementation of a disk I/O subsystem appropriated for the management of hard real time disk traffic. The proposed disk I/O subsystem allows the invocation of preemptive priority based disk I/O operations, both for non contiguous and contiguous data layouts. Disk I/O requests are scheduled in a common layer upon the disk driver, where access to the disk is enforced by either giving control or blocking invoking threads according to the selected scheduling policy (Rate Monotonic or EDF). The highest priority thread is granted access to the disk and it makes use of the full disk bandwidth, enforcing preemption whenever a higher priority request is demanding service. Preemptibility in non contiguously allocated files is achieved with a granularity that equals the time to read/ write a file system block. Additionally, a splitting up mechanism, that splits disk I/O requests into smaller ones, improves preemptibility in contiguously allocated files. Besides the description of the implementation we include a comprehensive performance evaluation. We have also identified the particular conditions under which a set of requests is schedulable in a predictable fashion using the proposed disk I/O subsystem