RAF: A Random Access First Cache Management to Improve SSD-Based Disk Cache

Offering better performance for random access compared to conventional hard disks and providing larger capacity and lower cost than DRAM, NAND flash based SSDsare integrated in server storage hierarchy as a second tier of disk cache between DRAM and disks for caching more data from disks to meet the increasingly intensive I/O demands. Unfortunately, available hybrid storage architectures cannot fully exploit SSDs' potentials due to absorbing too much workload of disk tier, which results in excessive wear and performance degradation associated with internel garbage collection. In this paper, we propose RAF (Random Access First), an hybrid storage architecture that combines both of an SSD based disk cache and a disk drive subsystem. RAF focuses on extending the lifetime of SSD while improving system performance through providing priority to caching random-access data. In detail, RAF splits flash cache into read and write cache to service read/write requests respectively. Read cache only holds random-access data that are evicted from file cache to reduce flash wear and write hits. Write cache performs as a circular write-through log so as to improve system response time and simplify garbage collection. Similar to read cache, write cache only caches random-access data and flushes them to hard disks immediately. Note that, sequential access are serviced by hard disks directly to even the full workload between SSD and disk storage. RAF is implemented in Linux kernel 2.6.30.10. The results of experiments show that RAF can significantly reduce flash wear and improve performance compared with the state-of-art FlashCache architecture.