Shingled magnetic recording

Shingled magnetic recording (SMR) is a magnetic storage data recording technology used in hard disk drives (HDDs) to increase storage density and overall per-drive storage capacity. Conventional hard disk drives record data by writing non-overlapping concentric magnetic tracks (conventional magnetic recording [CMR], Perpendicular recording), while shingled recording writes new tracks that overlap part of the previously written magnetic track, leaving the previous track narrower and allowing higher track density. Thus, the tracks partially overlap similar to roof shingles. This approach was selected because, if the writing head is made too narrow, it cannot provide the very high fields required in the recording layer of the disk.

The overlapping-tracks architecture complicates the writing process, since writing to one track also overwrites an adjacent track. If adjacent tracks contain valid data, they must be rewritten as well. As a result, a single write request may trigger a cascade of rewrites over a sequence of adjacent tracks. To place an upper limit on the number of rewrites needed to serve a single write request, SMR drives are divided into many append-only (sequential) zones of overlapping tracks that need to be rewritten entirely when full, resembling flash blocks in solid-state drives. Device-managed SMR devices hide this complexity by managing it in the firmware, presenting an interface like any other hard disk. Other SMR devices are host-managed and depend on the operating system to know how to handle the drive, and only write sequentially to certain regions of the drive.