A Redundant Disk Array Architecture for Efficient Small Writes

Parity encoded redundant disk arrays provide highly reliable, cost effective secondary storage with high performance for reads and large writes. Their performance on small writes, however, is much worse than mirrored disks — the traditional, highly reliable, but expensive organization for secondary storage. Unfortunately, small writes are a substantial portion of the I/O workload of many important, demanding applications such as on-line transaction processing. This paper presents parity logging, a novel solution to the small write problem for redundant disk arrays. Parity logging applies journalling techniques to substantially reduce the cost of small writes. We provide a detailed analysis of parity logging and competing schemes — mirroring, floating storage, and RAID level 5 — and verify these models by simulation. Parity logging provides performance competitive with mirroring, the best of the alternative single failure tolerating disk array organizations. However, its overhead is close to the minimum offered by RAID level 5. Finally, parity logging can exploit data caching much more effectively than all three alternative approaches. A Redundant Disk Array Architecture for Efficient Small Writes Daniel Stodolsky, Mark Holland, William V. Courtright II, and Garth A. Gibson October 20, 1993 CMU-CS-93-200 This research was supported by the ARPA, Information Science and Technology Office, under the title “Research on Parallel Computing”, ARPA Order No. 7330, the National Science Foundation under contract NSF ECD-8907068, and by IBM and NCR graduate fellowships. Work furnished in connection with this research is provided under prime contract MDA972-90-C-0035 issued by ARPA/CMO to CMU. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. government.