Flash-Aware High-Performance and Endurable Cache

Abstract

Flash-based SSDs are widely used as storage caches, which can benefit from both the higher performance of SSDs and lower price of disks. Unfortunately, issues of reliability and limited lifetime limit the use of Flash-based cache. One way to solve this problem is to use the flash memory as read cache and use other devices like nonvolatile memory for write buffering. In this paper, we propose a new flash-aware read cache architecture, which leverages out-of-place update property of flash memory to improve both cache hit ratio and lifetime. Due to the out-ofplace update property, when a cache entry is evicted from the flash cache, the eviction only removes the metadata, while the real data is still accessible and resides in the physical flash page until the whole flash block being erased. The main idea of our flash-aware cache is to reuse these evicted but still available data, when a request for the previously evicted data arrives, instead of accessing underlying storage to fetch the data and rewriting it into flash cache, we just need to revive the evicted data. To evaluate the benefits of flash-aware cache design, we implemented the normal LRU and flash-aware LRU (FLRU) cache algorithms on the Disksim simulator with an SSD extension. Our simulation results demonstrate that our flash-aware cache can improve the cache hit ratio by up to 28% and alleviate the lifetime limitation of flash cache by reducing the erase count by up to 70%.