Victim Disk First: An Asymmetric Cache to Boost the Performance of Disk Arrays under Faulty Conditions

The buffer cache plays an essential role in smoothing the gap between the upper-level computational components and the lower-level storage devices. A good buffer cache management scheme should be beneficial to not only the computational components, but also to the storage components by reducing disk I/Os. Existing cache replacement algorithms are well optimized for disks in normal mode, but inefficient under faulty scenarios, such as a parity-based disk array with faulty disk(s). To address this issue, we propose a novel asymmetric buffer cache replacement strategy, named Victim (or faulty) Disk(s) First (VDF) cache, to improve the reliability and performance of a storage system consisting of a buffer cache and disk arrays. The basic idea is to give higher priority to cache the blocks on the faulty disks when the disk array fails, thus reducing the I/Os directed to the faulty disks. To verify the effectiveness of the VDF cache, we have integrated VDF into two popular cache algorithms LFU and LRU, named VDF-LFU and VDF-LRU, respectively. We have conducted extensive simulations as well as a prototype implementation. The simulation results show that VDF-LFU can reduce disk I/Os to surviving disks by up to 42.3% and VDF-LRU can reduce those by up to 36.2%. Our measurement results also show that VDF-LFU can speed up the online recovery by up to 46.3% under a spare-rebuilding mode with online reconstruction, or improve the maximum system service rate by up to 47.7% under a degraded mode without a reconstruction workload. Similarly, VDF-LRU can speed up the online recovery by up to 34.6%, or improve the system service rate by up to 28.4%.