Track-based Translation Layers for Interlaced Magnetic Recording

Interlaced magnetic recording (IMR) is a state-of-the-art recording technology for hard drives that makes use of heat-assisted magnetic recording (HAMR) and track overlap to offer higher capacity than conventional and shingled magnetic recording (CMR and SMR). It carries a set of write constraints that differ from those in SMR: “bottom” (e.g. even-numbered) tracks cannot be written without data loss on the adjoining “top” (e.g. odd-numbered) ones. Previously described algorithms for writing arbitrary (i.e. bottom) sectors on IMR are in some cases poorly characterized, and are either slow or require more memory than is available within the constrained disk controller environment.

We provide the first accurate performance analysis of the simple read-modify-write (RMW) approach to IMR bottom track writes, noting several inaccuracies in earlier descriptions of its performance, and evaluate it for latency, throughput and I/O amplification on real-world traces. In addition we propose three novel memory-efficient, track-based translation layers for IMR—track flipping, selective track caching and dynamic track mapping, which reduce bottom track writes by moving hot data to top tracks and cold data to bottom ones in different ways. We again provide a detailed performance analysis using simulations based on real-world traces.

We find that RMW performance is poor on most traces and worse on others. The proposed approaches perform much better, especially dynamic track mapping, with low write amplification and latency comparable to CMR for many traces.