Shingled Magnetic Recording (SMR) and Two-Dimensional Magnetic Recording (TDMR)

Abstract

Data areal-density has always been a key metric in magnetic recording. An increase in ‘bits per square inch’ allows a higher capacity product to be developed at similar cost. Areal densities have grown at rates as high as 100% per year, but by 2010 these rates had dropped to more like 10%. In response to this slowdown, numerous other avenues were explored including increasing the number of disks in the form factor - an opportunity enabled by the advent of helium-filled drives. Alongside such mechanical improvements, two important innovations were made in the physical recording processes. These innovations were Shingled Magnetic Recording (SMR) and Two Dimensional Magnetic Recording (TDMR). SMR was developed as a response to the difficulty of maintaining fields while writing very narrow tracks with a conventional magnetic head. By keeping the head relatively wide, the field could be kept strong and by overlapping the written tracks, the final track pitch could be reduced. Two Dimensional Magnetic Recording (TDMR) was developed recognizing that increases in track density were also limited by the interference between tracks. By reading back the wanted track(s) with several closely-spaced sensors, the effects of the interference could be greatly mitigated. Both SMR and TDMR are now widely deployed in modern hard disk drives although neither realized the gains originally anticipated.