H-ARC: A non-volatile memory based cache policy for solid state drives

With the rapid development of new types of nonvolatile memory (NVM), one of these technologies may replace DRAM as the main memory in the near future. Some drawbacks of DRAM, such as data loss due to power failure or a system crash can be remedied by NVM's non-volatile nature. In the meantime, solid state drives (SSDs) are becoming widely deployed as storage devices for faster random access speed compared with traditional hard disk drives (HDDs). For applications demanding higher reliability and better performance, using NVM as the main memory and SSDs as storage devices becomes a promising architecture. Although SSDs have better performance than HDDs, SSDs cannot support in-place updates (i.e., an erase operation has to be performed before a page can be updated) and suffer from a low endurance problem that each unit will wear out after certain number of erase operations. In an NVM based main memory, any updated pages called dirty pages can be kept longer without the urgent need to be flushed to SSDs. This difference opens an opportunity to design new cache policies that help extend the lifespan of SSDs by wisely choosing cache eviction victims to decrease storage write traffic. However, it is very challenging to design a policy that can also increase the cache hit ratio for better system performance. Most existing DRAM-based cache policies have mainly concentrated on the recency or frequency status of a page. On the other hand, most existing NVM-based cache policies have mainly focused on the dirty or clean status of a page. In this paper, by extending the concept of the Adaptive Replacement Cache (ARC), we propose a Hierarchical Adaptive Replacement Cache (H-ARC) policy that considers all four factors of a page's status: dirty, clean, recency, and frequency. Specifically, at the higher level, H-ARC adaptively splits the whole cache space into a dirty-page cache and a clean-page cache. At the lower level, inside the dirty-page cache and the clean-page cache, H-ARC splits them into a recency-page cache and a frequency-page cache separately. During the page eviction process, all parts of the cache will be balanced towards to their desired sizes.