2F: A Special Cache for Mapping Table of Page-Level Flash Translation Layer

The development of flash memory drives flash based SSDs to enter into enterprise-scale storage systems. As the kernel of SSD, flash translation layer (FTL) attracts many attentions. Generally, there are two types of FTLs according to the granularity of address mapping: block-level and page-level mapping FTLs. We focus on the latter one. Typically, page-level mapping scheme must employ a cache to alleviate the memory pressure introduced by the big mapping table. We argue that classic cache replacement policies aren't competent for the page table cache of FTLs. The major contribution of this work is to design a dedicated cache replacement policy called Two Filters (abbreviated as 2F) for page-level mapping FTLs. 2F aims at two goals. The first is higher hit ratio as all the replacement policies pursue. As 2F not only protects frequently accessed pages, but also protects sequentially accessed pages at little cost, it does achieve a higher hit ratio. The second goal is to distinguish hot pages from the cold. This goal is special for page table of FTLs. If hot and cold pages are directed to separate blocks, garbage collection will be more efficient. In order to achieve this goal, 2F employs two filters. One is used for containing sequentially accessed pages. Another is used for selecting hot pages. Trace driven simulations present that 2F outperforms classic replacement policies in both hit ratio and data classification.