Leveraging Progressive Programmability of SLC Flash Pages to Realize Zero-overhead Delta Compression for Metadata Storage

This paper presents a method to implement delta compression for metadata storage in flash memory. With the abundant temporal redundancy in metadata, it is very intuitive to expect flash-based metadata storage can significantly benefit from delta compression. However, straightforward realization of delta compression demands the storage of the original data and the deltas among different versions in different flash memory physical pages, which leads to significant overhead in terms of read/write latency and data management complexity. Through experiments with 20nm NAND flash memory chips, we observed that, when operating in SLC mode, flash memory page can be programmed in a progressive manner, i.e., different portion of the same SLC flash memory page can be programmed at different time. This motivates us to propose a simple design approach that can realize delta compression for metadata storage without latency and data management complexity overheads. The key idea is to allocate SLC-mode flash memory pages for metadata, and store the original data and all the subsequent deltas in the same physical page through progressive programming. Experimental results show that this approach can significantly reduce the metadata write traffic without any latency overhead.