Balancing disk access times in RAID5 disk arrays in degraded mode by conditionally prioritizing fork/join requests

Abstract

RAID5 disk arrays with rotated parities can tolerate single disk failures by reconstructing missing blocks on demand by XORing the contents of corresponding K blocks on surviving disks by a K-way Fork/Join (F/J) request, which is considered completed after the K disks are accessed. F/J accesses in RAID5 are processed concurrently with interfering disk accesses. The mean response time of F/J and independent/interfering requests: R F/J K and R Ind and the mean delay from the completion of the first to the last F/J task, known as task dispersion time: T disp K , are performance metrics of interest. Given R F/J K > R Ind with FCFS scheduling, it is desirable to equalize disk access times, but giving a higher nonpreemptive priority to disk accesses due to F/J requests with respect to interfering disk accesses results in R Ind > R F/J K . We propose a continuum of conditional priority methods based on the fraction F of F/J accesses completed with FCFS scheduling. F = ∞ stands for FCFS and F = 0 stands for unconditional priorities. Simulation shows that F = 1/8 with K = 8 yields R F/J K ≈ R Ind for three distributions of disk requests and in the range of F/J and independent disk requests considered. F can be varied adaptively based on measurement results to balance disk access times.