Latent Sector Faults and Reliability of Disk Arrays
Hannu H. Kari · Aaltodoc (Aalto University) · 2005
This thesis studies the effects of latent sector faults on reliability, performance, and combined performability of disk arrays. This is done by developing two novel reliability models that include two fault types: disk unit faults and sector faults. The new reliability models study both hot swap and hot spare approaches in repairing disk unit faults. A traditional analytical method and also a novel approximation method are used for analyzing these reliability models. Two RAID (Redundant Array of Inexpensive Disks) configurations are used in the analysis: RAID-1 and RAID-5. Significant drop in reliability and performability is resulted when latent sector faults are not quickly detected. A sector fault may stay a long time undetected if user’s disk access pattern is unevenly distributed and the fault resides on a seldom accessed area. To reduce the risk of latent sector faults, this thesis proposes four novel disk scanning algorithms that utilize the idle time of disks to read periodically the entire disk surface in small segments. The main idea of the disk scanning algorithms is the same as in the memory scrubbing algorithm, but this is the first time when this approach is used with the secondary memory. The disk scanning algorithms are analyzed in this thesis and dramatic improvement in reliability and performability is achieved while there is only a minor effect on performance.