Microelectromechanical systems MEMS based storage

1
Microelectromechanical systems (MEMS) based
storage

• Jan Prokaj

2
Overview

• Disk Drive architecture
• What is MEMS-based storage
• How does it work
• Compare with hard disks
• MEMS Performance
• Summary

3
Disk Drive Architecture

• Organized into platters, tracks, and sectors
• Track at the same position on every platter is a
  cylinder

4
MEMS-based storage

• Hybrid of semiconductor memory and disk drive
• Silicon wafer fabricated circuit and mechanically
  positioned recording heads
• Offers decreased cost, access time, size, power
  dissipation, failure rate, and shock sensitivity
• These devices can integrate computation with
  storage

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MEMS

• Developed at Carnegie Mellon, IBM, HP

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MEMS

• Recording heads are probe tips (fixed)
• Variations in media surface require tip height
  control (cantilevers)
• The media sled is spring-mounted above a an array
  of these probe tips
• It slides in X, Y directions (no rotation)
• Force applied by electrostatic actuators along
  each edge

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MEMS Data Access

• The media sled is first pulled to a specific
  location (x,y displacement)
• Seek
• Then the sled is
  moved at a constant
  velocity in the Y
  direction while data
  is read or written by
  the stationary probe
  tips

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Device Size example

• 196 mm2 total footprint
• 64 mm2 usable area, 6400 probe tips
• 40x40 nm bit cells (square, unlike in disk
  drives)
• 2 bit / byte overhead (encoding and ECC)
• 3.2 GB capacity
• 15-30 times greater aerial density than disk
• Smaller mass allows random 4 KB access time less
  than 1 ms

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MEMS Data Layout

• Media divided into rectangular regions
• Each region accessible by one probe tip

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MEMS Data Layout
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Device Layout example

• Each region 2500x2500 bits
• 2500 cylinders each holding 1350 KB
• 6400 total tips, 1280 active ? 5 tracks in each
  cylinder (270 KB each track)
• Accessing entire track takes 3.47 ms
• Each track has 34,560 8-byte sectors, of which
  1280 accessed in parallel (10 KB)
• Smallest accessible unit, 0.129 ms access
• 64 sectors group into logical block (512 B)

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MEMS

• Y dimension access speed is determined by the
  per-tip data rate, bit cell width, and sled
  actuator force
• 28 mm/s, 0.7 Mb/s per tip
• Moving in X dimension requires extra settling
  time (caused by spring oscillations)
• When switching tracks, the sled performs
  turnaround (28 mm/s to -28 mm/s), extra seek time

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MEMS Comparison to disks

• Mechanical positioning
• Seek and rotation for disk
• X,Y seek for MEMS
• Settling time
• Small component for disks (0.5 ms of 1-15 ms)
• Large component for MEMS (0.2 ms of 0.2-0.8 ms)

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MEMS Comparison to disks

• Seek time vs. Seek distance
• For disks, seek time is relatively constant over
  seek distance
• For MEMS, seeks near edges take longer than near
  the center (spring restoring forces)
• Recording density
• Same magnetic recording technology
• MEMS higher bit density means more defective bits

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MEMS Comparison to disks

• Numbers of mechanical components
• MEMS has many more parts, though robust
• More likely some will break
• Concurrent read/write heads
• Most disks use 1 head at a time
• MEMS uses thousands of probe tips concurrently ?
  increased bandwidth
• Startup activities
• MEMS starts up in much less time and requires
  much less power than disk

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Performance of MEMS

• DiskSim storage simulator
• An order-of-magnitude improvement in systems
  average service time (0.52 ms versus 10.1 ms for
  disk)
• X seek time is the dominant factor
• Track switching time depends on access velocity
  (turnaround time)
• MEMS 3x faster in PostMark benchmark (much faster
  positioning time)

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IBM Millipede

• Thousands of tips punch indentations representing
  individual bits into a thin plastic film
• Rewritable
• Trillion bits / inch2

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Summary

• High bit density
• But fault management important
• Fast access time
• High bandwidth
• Low power usage
• Ideal for PDAs, and other low power devices

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References

• Carley, L.R., Ganger, G.R., and Nagle, D.F.
  MEMS-based Integrated-Circuit Mass-Storage
  Systems. Communications of the ACM, Vol. 43
  (2000), 73-80.
• Griffin, J.L., Schlosser, S.W., Ganger, G.R., and
  Nagle, D.F. Operating System Management of
  MEMS-based Storage Devices. Proceedings of the
  4th Symposium on Operating Systems Design and
  Implementation, 2000.
• Hennessy, J., and Patterson, D. Computer
  Architecture A Quantitative Approach. San
  Francisco MKP, 2003.
• Hard Disk Drive Basics. http//www.ntfs.com/hard-d
  isk-basics.htm
• IBM Research. IBMs Millipede Project
  Demonstrates Trillion-bit Data storage density.
  http//domino.research.ibm.com/
  comm/pr.nsf/pages/news.20020611_millipede.html