WAFLASH: Taming Unaligned Writes in Solid-State Disks

Ahstract-NAND-flash based solid-state disks (SSDs) are replacing the hard-disk drives (HDDs) in various storage systems from high-end servers in data centers to mobile computers on the edges of cloud computing. Due to the architectural nature of SSDs, performance-sensitive applications may generate a large number of unaligned writes on SSDs, which can cause many issues including chip congestion, sub-request blocking, chip load imbalance, and write space amplification. This paper presents a Write-Aligned FLASH drive (WAFLASH) that comprehensively and significantly alleviates the side-effects of unaligned writes in solid-state disks. We utilize three key techniques: (1) prioritizing eviction of fully-filled pages over partially-filled pages in write buffers, (2) storing multi-version data requested by unaligned writes/overwrites in flash memory, and (3) compacting multiple small partially-filled pages in a physical page to circumvent write amplification and reduce the number of additional reads in the critical I/O path. We implement WAFLASH in VSSIM and Cosmos+ OpenSSD. The results show WAFLASH increases I/O throughput by up to 125% with the Filebench benchmark.