Cap: Exploiting Data Correlations to Improve the Performance and Endurance of SSD RAID

Parity-based RAID provides system-level fault tolerance. However, parity updates caused by small writes introduce lots of extra I/Os, degrading I/O performance and wearing SSDs out. It has been proposed to use Non-Volatile Memory (NVM) as a parity cache on an SSD RAID to postpone parity updates until the whole stripe has been updated. However, this often fails because of skewed distribution of hot data chunks within a stripe. In real workloads, it is often difficult to achieve a full-stripe update even after a long delay. In this paper, we propose a Correlation aware parity caching scheme, called Cap, for SSD-based RAIDs. The key idea behind Cap is to periodically reconstruct correlated hot data chunks into a new stripe. Since these data chunks have a strong correlation, they tend to be updated together within a short time span. This co-update within a stripe more efficiently utilizes the parity cache to convert partial-stripe updates into a full-stripe update. We have implemented Cap on a RAID-5 SSD array in Linux Kernel 4.3. Experimental results show that Cap improves the I/O bandwidth by 54%~145% compared with the Linux software RAID. Compared with the state-of-the-art parity caching scheme PPC, Cap improves the I/O bandwidth by 14%~31%.