A Shingle-Aware Persistent Cache Management Scheme for DM-SMR Disks

Shingled Magnetic Recording (SMR) Technology is a new promising disk technology, which enables high areal density for disk storage by adopting writing tracks in an overlapped manner. Due to track overlapping, however, SMR devices cannot support in-place updates on shingled disk tracks, as overwriting shingled tracks would damage previously written data on neighboring tracks, resulting in poor random write performance. Drive-managed SMR (DM-SMR) devices attempt to circumvent this idiosyncrasy by deploying an internal persistent cache which absorbs incoming writes temporally and persists them later on in batches, while providing backward compatibility by reserving the same block access interface. The persistent cache management policy has to maintain the mapping associations between fresh writes in the persistent cache and their target destinations on the disk, resulting in a so-called shingle translation layer (STL). Leveraging the sequential-only write property of shingled devices, in this paper, we propose a new shingle aware persistent cache cleaning policy for DM-SMR drives. Unlike traditional management polices, our new policy first merges cached updates by flushing writes that can be safely written to the disk, i.e., in the shingle direction, so that the cache space can be freed without paying the cost of read-modify-write. It then defaults to the normal cache merging process if it needs to reclaim more cache space. Our evaluations have shown that our persistent cache management policy delivers better performance (by up to 2.5X) via dramatically reducing write amplification associated with persistent cache cleaning and alleviating fragmented reads.