Reducing Write Amplification of DM-SMR Disks by Shingle-aware Persistent Cache

Shingled Magnetic Recording (SMR) technology is a cutting-edge disk tech that boosts areal density via overlapping track writing. However, track overlapping makes SMR devices unable to support in place updates on shingled tracks, risking data corruption and degrading random write performance. DM-SMR devices use an internal persistent cache to absorb writes and persist them later on in batches during the garbage collection process to maintain compatibility. In this paper, we propose a new shingle aware persistent cache cleaning policy for DM-SMR drives. Unlike traditional management policies, 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 incurring the cost of read-modify-write. In order to proactively free up more cache space at once, we adopt a delayed write-back strategy by recording the data blocks that can be safely written back to the disk, as well as the dependency relationships between them. Through dynamically and proactively releasing cache space, the frequency of garbage collection can be effectively reduced. Our evaluations have shown that our persistent cache management policy delivers better performance (by up to 3.3X) via dramatically reducing write amplification associated with persistent cache cleaning and alleviating fragmented reads.