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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
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