PARITY-LOGGING DISK ARRAYS

Parity-encoded redundant disk arrays provide highly reliable, cost-eff ective secondary storage with high performance for reads and large wri tes. Their performance on small writes, however, is much worse than mi rrored disks-the traditional, highly reliable, but expensive organizat ion for secondary storage. Unfortunately, small writes are a substanti al portion of the I/O workload of many important, demanding applicatio ns such as on-line transaction processing. This paper presents parity logging, a novel solution to the small-write problem for redundant dis k arrays. Parity logging applies journalling techniques to substantial ly reduce the cost of small writes. We provide detailed models of pari ty logging and competing schemes-mirroring, floating storage, and RAID level 5-and verify these models by simulation. Parity logging provide s performance competitive with mirroring, but with capacity overhead c lose to the minimum offered by RAID level 5. Finally, parity logging c an exploit data caching more effectively than all three alternative ap proaches.