μCache: a mutable cache for SMR translation layer

Abstract

Shingled Magnetic Recording (SMR) may be combined with conventional (re-writable) recording on the same drive; in host-managed drives shipping today this capability is used to provide a small number of re-writable zones, typically totaling a few tens of GB. Although these re-writable zones are widely used by SMR-aware applications, the literature to date has ignored them and focused on fully-shingled devices. We describe μCache, an SMR translation layer (STL) using re-writable (mutable) zones to take advantage of both workload spatial and temporal locality to reduce the garbage collection overhead resulted from out-of-place writes. In μCache the volume LBA space is divided into fixed -sized buckets and, on write access, the corresponding bucket is copied (promoted) to the re-writable zones, allowing subsequent writes to the same bucket be served in - place resulting in fewer garbage collection cycles. We evaluate μCache in simulation against real-world traces and show that with appropriate parameters it is able to hold the entire write working set of most workloads in re-writable storage, virtually eliminating garbage collection overhead. We also emulate μCache by replaying its translated traces against actual drive and show that 1) it outperforms its examined counterpart, an E-region based translation approach on average by 2x and up to 5.1x, and 2) it incurs additional latency only for a small fraction of write operations, (up to 10%) when compared with conventional non-shingled disks.