Shingled Magnetic Recording

Parag Panda

panda025@umn.edu

Comparison of Storage media devices

Courtesy : ICT Lounge

Courtesy : ICT Lounge

Market Segmentation

Courtesy : SNIA-Japan Cold Storage Seminar

• Solid-state storage devices such as NAND flash are dramatically changing the playing field for durable storage systems, especially for smaller, randomly accessed, performance-sensitive storage systems.

• But, bulk of online stored information will be magnetically recorded on hard disks because of the small size of magnetically recorded bits and the low cost for a device containing dozens of terabits.

• Consumer expectations for ever larger capacity magnetic disk drives, and the economics of the magnetic disk drive marketplace, call for an annual aggressive increase in areal density (more than 30% per year, recently about 40% per year).

• Maintaining this rate of increase in areal density in the face of the impending superparamagnetic limit is the core challenge of magnetic recording technologists today.

• New recording system technologies are needed to keep the HDD industry on its historical track of delivering capacity improvements overtime.

Motivation

Recording System Requirements•Center the recording head above the data track.

• Fly the head very close to the recording medium.

•Write sharp transitions in the recording medium.

• Store the data reliably for many years.

•Read the data back with high SNR and high resolution.

•Decode the signal with very read back errors.

•All of these with high reliability, high performance, low power, and for low price/GB.

The Recording Mechanism

Courtesy : Hitachi Global Storage Technologies

Magnetic Spacing Challenges

Courtesy : Hitachi Global Storage Technologies

Scaling Challenge of Conventional Recording• A larger write head addresses the scaling challenge of conventional magnetic

recording

Courtesy : IDEMA, SMR

Media “Trilemma”• Superparamagnetic effect forces a trade-off for media design

Courtesy : Western Digital

Magnetic Recording System Techniques• New recording system technologies are needed to keep the HDD industry on its

historical track of delivering capacity improvements over time.

Longitudinal Vs Perpendicular Recording• Longitudinal recording

aligned the poles of each magnetic element (bit) horizontally.

• PMR aligns the poles vertically.

• Aligning the poles vertically allows the use of magnetically stronger materials (higher coercivity) by routing the magnetic flux through an additional under-layer.

What is Shingled Magnetic Recording?• Shingling overlaps tracks

written sequentially by a wider writer.

•Residual tracks narrower than originally written widths.

• Shingled writing is not new. Long used in WORM tape storage.

Courtesy : Western Digital

What is Shingled Magnetic Recording?•Conventional write heads need to write a narrow track with

sharply defined edges on both sides, tolerant of the skew introduced by different angular positions (inner to outer diameters).

• Shingling means that the track written can be wider, allowing a stronger write field, only one edge needs to be sharp so it can be sharper. Of course, the read head has to read thinner tracks with much closer adjacent tracks causing interference.

SMR write head geometry extends well beyond the track pitch.

SMR Constraint

• No or little host modification required.

• Drive ensures data integrity and optimizes performance for a balanced set of typical IO workloads.

In SMR, the large write head overwrites adjacent, previously written tracks, creating a need for SMR management.

Courtesy : IDEMA, SMR

Advantages of Shingled Magnetic RecordingAdvantages:

• Stronger write field and gradient.

• No adjacent track erasure (ATE) due to multiple write brings further stronger field.• Stronger field brings improvement on linear

density.

• Sharp corner-edge field brings narrower erase band. • These enables us to increase track

density (TPI).

• TPI gain expected even on conventional head at higher skew angle. Courtesy : Western Digital

Head Design• Writer design for corner writing with higher write fields – asymmetric

writer for high side-field gradients.

• Reader driven capability - reader response and side reading is more important now.

Areal Density• Higher areal density with current component technology.

• 2 Tbits/sq.in. or more possible with continuous media.

Conventional Vs SMR Writing

Courtesy : SNIA, SMR Models, Standardization and Applications

Updating a band with new data

Courtesy : SNIA, SMR Models, Standardization and Applications

Updating a band with new data

Courtesy : SNIA, SMR Models, Standardization and Applications

Updating a band with new data

Courtesy : SNIA, SMR Models, Standardization and Applications

Random Write Performance with Disk Cache

Courtesy : SNIA, SMR Models, Standardization and Applications

Summary of the Implementation•Write-back for random writes

• Large disk cache.• Fast response for bursty workloads.• Aggregation of multiple commands before band update.

•Write-around for sequential writes• Conventional performance at media data rate.

•No host changes required• Performance similar to conventional drives in client benchmarks

Best Practices•Reads• Same as non-SMR

•Writes•Maximize long sequential write runs.

• Align to 4KB physical sectors.• Limit number of interleaved sequential writes.

• Limit and concentrate random writes

Evolution of SMR• An SMR-aware host stack may be able to further optimize the system IO

performance.

Courtesy : IDEMA, SMR

Host Managed SMR• Open source software developers kit is available at:

• http://github.com/hgst

• Allows Linux apps access to Host-Managed SMR HDD

• Ensures new command sets flow through HBA

• Emulates Host Managed SMR on PMR drives.

Storage Architecture (SJRC & JRL) HGST Research

Storage Architecture (SJRC & JRL) HGST Research

Storage Architecture (SJRC & JRL) HGST Research

References• http://ewh.ieee.org/r6/scv/mag/MtgSum/Meeting2010_10_Presentation.pdf

• http://www.storagereview.com/what_is_shingled_magnetic_recording_smr

• http://www.snia.org/sites/default/files/Dunn-Feldman_SNIA_Tutorial_Shingled_Magnetic_Recording-r7_Final.pdf

• http://www.idema.org/wp-content/downloads/2212.pdf

• http://www.tomsitpro.com/articles/shingled-magnetic-recoding-smr-101-basics,2-933.html

• http://www.cs.cmu.edu/~garth/papers/05_feldman_022-030_final.pdf

• http://www.pdl.cmu.edu/PDL-FTP/Storage/CMU-PDL-11-107.pdf

• http://www.idema.org/wp-content/downloads/2234.pdf

• http://asia.stanford.edu/events/spring08/slides402S/0410-Dasher.pdf

• http://events.linuxfoundation.org/sites/events/files/slides/SMR-LinuxConUSA-2014.pdf

• http://snia-j.org/docs/tech/20150709_SNIA_COLDSTORAGE6.pdf