Caching less for better performance: Balancing cache size and update cost of flash memory cache in hybrid storage systems

Hybrid storage solutions use NAND flash memory based Solid State Drives (SSDs) as non-volatile cache and traditional Hard Disk Drives (HDDs) as lower level storage. Unlike a typical cache, internally, the flash memory cache is divided into cache space and over-provisioned space, used for garbage collection. We show that balancing the two spaces appropriately helps improve the performance of hybrid storage systems. We show that contrary to expectations, the cache need not be filled with data to the fullest, but may be better served by reserving space for garbage collection. For this balancing act, we present a dynamic scheme that further divides the cache space into read and write caches and manages the three spaces according to the workload characteristics for optimal performance. Experimental results show that our dynamic scheme improves performance of hybrid storage solutions up to the off-line optimal performance of a fixed partitioning scheme. Furthermore, as our scheme makes efficient use of the flash memory cache, it reduces the number of erase operations thereby extending the lifetime of SSDs.