An Introduction to Disk Drive Modeling Chris Ruemmler

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An Introduction to Disk Drive ModelingChris
Ruemmler John WilkesHewlett-Packard
Laboratories

• Presented by Hang Zhao

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The earliest hard disks

• First Hard Disk (1956) IBM's RAMAC, capacity is
  5 MB, stored on 50 24" disks
• First Air Bearing Heads (1962) IBM's model 1301
  increases both areal density and throughput by
  about 1000
• First Removable Disk Drive (1965) IBM's model
  2310

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A brief history of hard disk drive

• Engineers over the last few decades have done at
  improving them in every respect reliability,
  capacity, speed, power usage, and more.

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Evolution of areal density of hard disk platters

• The areal density of hard disk platters continues
  to increase at an amazing rate even exceeding
  some of the optimistic predictions of a few years
  ago. Modern disks are now packing as much as 20
  GB of data onto a single 3.5" platter!

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Why do we need to model disk drive behavior?

• CPU technology is advancing rapidly while the
  overall system behavior is restrict to disk
  system performance.
• The behavior of disk drive itself is a dominant
  factor in overall I/O performance.
• Existing hard disk models have limitations

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Characteristics of modern disk drives

• Mechanism
• recording component
• positioning component
• Controller
• microprocessor
• buffer memory
• interface to SCSI bus

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Disk Drive Terminology

• Several platters, with information recorded
  magnetically on both surfaces (usually)

• Bits recorded in tracks, which in turn divided
  into sectors

• Actuator moves head (end of arm,1/surface) over
  track (seek), select surface, wait for sector
  rotate under head, then read or write
• Cylinder all tracks under heads

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The recording components

• Modern disks range in size from 1.3 to 8 inches
  in diameter
• Smaller disks VS larger disks
• less surface area/storage
• consume less power
• spin faster
• smaller seek distance

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The increased storage density

• The incremental trends result from
• better linear recording density a measure of how
  tightly the bits are packed within a length of
  track. (50,000 BPI 1994 524,000 BPI 2000)
• packing separate tracks more closely together
  (20,000 TPI 67,300 TPI around 2000)

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Platters and disk rotation

• Platters rotates in lockstep on a central spindle
  at rates varying from 3,600 to 7,200 rpm
• Higher spin rate increases transfer rates and
  shortens rotation latencies on the other hand,
  power consumption increases
• Each platter surface is associated with a disk
  head for writing and reading operating under a
  single read-write channel

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The positioning components

• Seeking speed of head movement, limited by the
  power available for pivot and the arms
  stiffness. Seek time is composed of
• speedup
• coast for long seeks where arms move at max?
• slowdown
• settle dominant factor of very short seeks

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The demerit of average seek time

• Average seek times are commonly used as a
  figure of merit for disk drives, but they can be
  misleading.
• Independent seeks are rare in practice.
• Shorter seeks are much more common.
• The one-third-stroke calculation is only
  applicable for completely independent seeks.
• N-1 weighted seek time calculation provides
  better approximation.
• Seek-time-versus-distance profile matters for
  modeling!

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The track following system

• Fine-tuning the head position at the end of a
  seek and keeping the head on the desired track.
• Performing a head switch from one surface to the
  next in the same cylinder.
• Aggressive and optimistic approach applied to
  head settling before a read operation.

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Data layout in SCSI disk

• Disk appears to client as a linear vector of
  addressable blocks, which are mapped to physical
  sectors on the disk.
• Zoning adjacent cylinders are grouped into zones
• Track skewing logical sector zero on each track
  is skewed for fast sequential access across track
  and cylinder boundaries.
• Sparing map flawed sectors to other locations

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The disk controller

• Mediates access to the mechanism
• Runs the track-following system
• Transfers data between disk drive and its client
• Manages the embedded cache

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Bus Interfaces
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Caching requests

• Read ahead
• Write caching
• Command queuing

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Modeling disk drives

• Disk drive cannot be modeled analytically with
  any accuracy due to its nonlinear,
  state-dependent behavior.
• Limitations for current modeling strategies
• Seek times modeled as a linear function of seek
  distance
• Rotational latency follows uniform distribution
• Media transfer time ignored or as fixed constant
• Bus contention often ignored

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The simulator and traces

• Event-based simulator in C
• Disk drive is modeled as two tasks and some
  additional control structure
• Representative samples from a longer trace series
  of HP-UX was selected
• Two HP disk drives
• HP C2200A for non-caching disk drive
• HP 97560 for caching disk drive

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Simulation model structure
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Evaluation

• Metric the root mean square of the horizontal
  distance between real drive curve and model curve
• The demerit was presented in both absolute term
  and relative term

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Evaluation cont.

• Non-linear seek time profile is added to the 3rd
  model in Figure c.
• The cost of head and track switching was also
  included
• Rotational latency and spare sector placement
  were added to the final model in Figure d.

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Evaluation cont.
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Summary of disk drive model

• Factors not included in the final model
• Soft-error reentries
• Individual spared sectors or tracks

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Impact of proposed disk drive model

• 269 citations found with most recent reference
  Including Models of Black-Box Storage
  Arrays-Terence Kelly Ira (2004)
• A Stochastic Disk I/O Simulation
  Technique, Winter Simulation
  
  Conference, 1997
• Modeling hard disk power
  consumption by Princeton,
  published in FAST 03