T-STL: Track-based Shingled Translation Layer for Autonomous SWD

Shingled Magnetic Recording (SMR) is a promising technique for increasing hard disk drive (HDD) capacity by overlapping adjacent tracks, which enforces writes to be performed in a sequential manner in order not to destroy the valid data in the following neighboring tracks. In this paper we introduce T-STL, a Track-based Shingled Translation Layer for autonomous Shingled Write Disks (SWDs). T-STL minimizes the write amplification overhead by utilizing two unique properties of SWDs to handle workloads differently according to the SWD space utilization. First, when SWD space utilization is less than 50%, T-STL turns the SWD into a HDD-like device by using every other track 1. The unused tracks work as safety gaps to avoid data overwriting. Therefore, in-place updates are possible in this situation. The second property is a track-based mapping instead of a typical block-based mapping. When SWD utilization is over 50%, tracks that do not allow in-place updates are updated in a copy-on-write manner as out-of-place updates. In a track-based mapping, when a track is invalidated, it becomes free right away and can be immediately reused as long as the next track is free too, without triggering an explicit garbage collection (GC) operation. Only when the free SWD space becomes extremely fragmented, an explicit on-demand GC operation needs to be invoked to create big contiguous free space by migrating some valid tracks. Efficient track-level mapping and space management schemes are also designed to fully utilize these two properties. We implement the T-STL scheme and compare it with a regular HDD, an existing out-of-place update SWD design and an in-place update SWD. The experiments with several realistic traces and one synthetic trace demonstrate that the T-STL scheme can perform much better than the existing SWD designs and even nearly as good as regular HDDs when SWD space usage is less than 50%. ∗This work was partially supported by NSF Awards: 1217569 and 1439622. 1We assume the write head is 2-track wide in our discussions. However, our schemes can be adapted for bigger write head width.