shingled magnetic recording (SMR)

Shingled magnetic recording (SMR) is a technique for writing data to a hard disk drive (HDD) whereby the data tracks partially overlap to increase the areal density and overall storage capacity per disk.

The term SMR derives from the manner in which installers apply shingles to a roof, with portions of the shingles overlapping each other.

Conventional magnetic HDDs pack narrow concentric tracks in parallel as closely as possible to increase data density. The recorded signal must sharply diminish on both sides of a newly written track in order to leave a gap between tracks, and that task becomes more difficult as tracks get narrower and closer together. Manufacturers have to shrink the width of the magnetic poles on the write element in the read/write head to accommodate the narrow writing. The constriction of the poles has the unwanted consequence of reducing the strength of the magnetic field to write data to the disk.

SMR HDDs take a different approach to overcome the physical limitation of conventional hard drives. The head is relatively wide, and the SMR drive writes the data tracks strictly sequentially in overlapping fashion. The write width and pole-tip width can be much larger than the resulting final track width, or shingled width.

As a result, the system can use a relatively strong writer to create very narrow tracks and achieve greater data density. Since the data in the newly written track overlaps the data in the adjoining track, updates of individual tracks or sectors may require a read and rewrite of an entire band of shingled tracks.

The SMR disk's tracks need sharp definition on only the edge that is overlapping the previously written adjacent track. Design engineers thus have the freedom to make the critical edge sharper, allowing the shingled tracks to be placed closer together.

An SMR disk drive works optimally for long sequential writes, when the system writes data continuously to the disk. Examples of sequential writes include backups and archives of data. Random modifications to prewritten data on the disk can potentially cause inconsistent write throughput performance, but manufacturers have worked on SMR design and firmware and software improvements to address the issue.

Major manufacturers of SMR hard drives include Seagate and Western Digital's HGST division.

This was last updated in July 2015

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