Repair algorithms for mirrored disk systems

This paper analyzes different repair methods of a mirrored disk subsystem. The main interest is focused on disk faults and how the repair process is copying data from a fault-free disk to a spare disk with the least performance degradation for user disk requests. The objective of this study is to compare how different repair algorithms affect system performance. Two different repair algorithms are compared. Two different access patterns (uniform and non-uniform) are studied to establish their effects on the repair process and performance. Simulation results of this research indicate that the performance degradation of user disk requests can be significantly reduced by introducing a short delay in the repair algorithm. A new algorithm for detecting sector faults is also presented. This algorithm scans the disk space, while there are no user disk requests issued and detects deteriorated media using the advanced statistics of modern SCSI disks. The advantage of the proposed algorithm is that it can repair the disk subsystem before data is actually lost due to a media defect (bad sector).