Shingled Magnetic Recording (SMR) was named after roof shingles. With SMR, relatively wide tracks are written to the disk and successively written data tracks partially overlap the previous ones, similar to the manner roof shingles are applied.

Visual of Shingled Magnetic Recording

A key component of increasing data density in HDDs is making thinner data tracks and packing them closer together.

Writing narrower tracks has always meant narrowing the width of the poles of a tiny magnet on the read/write head. When energized, a magnetic field emanating from the poles writes and erases data by flipping the magnetization of small regions, called bits, on the spinning disk directly below. But narrowing the poles also reduces the strength of the writing magnetic field, so they can’t be shrunk to the point where the field can’t flip a bit’s magnetization.

One way to get the data-density benefit of narrower tracks without actually writing them is to write relatively wide tracks that partially overlap the previous ones. This technique is called shingled magnetic recording (SMR) because successively written data tracks overlap in a manner similar to roof shingles.

With careful design and execution, SMR is expected to enable data densities as high as 3 trillion bits per square inch. It is currently the plan-of-record for next-generation products across the industry.

In practice, SMR’s overlapping tracks are best suited for continuous writing/erasing rather than small random-access updates. In some cases, an ideal solution could be to designate part of a disk to have high-density shingled tracks for storing log files while the rest is not shingled to permit more rapid random access at a lower data density.

Additional Resources:

HGST Fellow Roger Wood’s Oct. 19, 2010, presentation to Santa Clara Chapter, IEEE Magnetics Society
Shingled Magnetic Recording and Two-Dimensional Magnetic Recording

“Shingled Recording for 2-3 Tbit/in2” by S. Greaves et al, IEEE Trans. Magnetics, Vol. 45, No.10, October 2009, 3823
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