Improving RAID Performance Using an Endurable SSD Cache

Parity-based RAID storage systems have been widely deployed in production environments. However, they suffer from poor random write performance due to the parity update overhead, i.e., small write problem. With the increasing density and decreasing price, SSD-based caching offers promising opportunities for improving RAID storage I/O performance. However, as a cache device, frequent writes to SSD leads to being quickly worn out, which causes high costs and reliability problems. In this paper, we propose an efficient cache management scheme by Keeping Data and Deltas (KDD) in SSD. KDD dynamically partitions the cache space into Data Zone (DAZ) and Delta Zone (DEZ). DAZ stores data that are first admitted into SSD. On write hits, KDD writes the data to RAID storage without updating the parity blocks. Meanwhile, the deltas between old version of data and the currently accessed data are compactly stored in DEZ. In addition, KDD organizes the metadata partition on SSD as a circular log to make the cache persistent with low overhead. We evaluate the performance of KDD via both simulations and prototype implementations. Results show that KDD effectively reduces the small write penalty while significantly improving the lifetime of the SSD-based cache.