COMP 206: Computer Architecture and Implementation

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COMP 206Computer Architecture and Implementation

• Montek Singh
• Mon, Nov 21, 2005 Mon, Nov 28, 2005
• Topic Storage Systems (Disk Technology)

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Disk Systems Characteristics

• Capacity bytes
• Mainframes typically have ?3.7 GB of disk storage
  per MIPS
• PCs rend to require ?5 MB of disk storage per
  MIPS
• Bandwidth (throughput)
• Bytes transferred per unit time
• Service rate
• Number of service requests satisfied per unit
  time
• Supercomputer I/O requests tend to involve large
  amounts of data transaction processing systems
  tend to involve small ones
• Response time (latency)
• Time between start and completion of an event
• Cost /MB

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Example Parameters of A Single Disk Drive
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Technical Details (1)

• Platters
• 2-4mm thick, made of an aluminum alloy
• Typically 1-6 platters in a hard disk
• Both sides of each platter generally used
• One side of one platter dedicated to information
  to guide the servomechanisms that control speed
  of rotation and head movement
• Each recording surface has its own read-write
  head
• Only one operational at any time
• Data transfer to and from disk is bit-serial
• Rotation
• Speed of rotation carefully controlled by
  servomechanism
• E.g., 7200 rpm ?1 or even ?1 rpm
• All disk drives are synchronized in some disk
  arrays
• Angular positions are identical (within
  tolerance) at any time

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Technical Details (2)

• Flying height
• Read/write heads do not touch rotating disk
  surface
• Careful aerodynamic design keeps small constant
  distance (?0.2 m)
• Disk moves with approximate linear speed of 700
  inches/sec
• 18m/s, 40 miles/hour
• Smaller flying height ? higher writing density
• Hence, platters and heads sealed hermetically in
  clean space
• Called Winchester drives for historical reasons
• Parking the heads
• Before disk is allowed to slow down and stop,
  heads are moved over an area of the disk not used
  for recording, where they are allowed to come
  into contact with the disk surface

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Why Disks Are Bit-Serial

• High TPI value makes parallel access disks
  unworkable
• Reading heads move as rigid unit
• One of them (the one over the servo disk)
  supposedly defines radial (track) position
• Ensemble is not truly rigid, and various reasons
  (like thermal dilations) prevent all heads being
  positioned over same track simultaneously,
  repeatedly, and reliably
• Only one head is positioned accurately at a time
  servo guides the assembly to approximate position
  of requested track, reading head does final
  positioning using a high frequency signal between
  data tracks
• Sector ID always contains track number for
  confirmation
• Portable disks use shock sensors to prevent
  overwriting of adjacent tracks caused by jarring
  of R/W head

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Track Densities

• Until recently, only innermost track was recorded
  with maximum density
• All other tracks contained same number of sectors
  and bytes
• Recently, manufacturers are using bands of tracks
• Total number of tracks (100s-1000s) divided into
  bands or zones (4, 8, 16, ) each containing the
  same number of tracks
• Each band has innermost track recorded at maximum
  density, with other tracks having same capacity
• Greatest capacity gains occur with a small number
  of bands
• Two scheduling options
• Constant rotational speed, use buffer for speed
  matching
• Constant data rate, head spinning with rotational
  speed corresponding to recording density of zone

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Sector Format

• Sector is the smallest unit of data that can be
  read or written
• Typically between 32B and 4KB, with 512B being a
  common size
• Format of sector
• ID Identifies sector with information such as
  angular position and track
• Has its own error correcting code (ECC)
• GAP Permits electronics to process ECC
  information
• DATA and its ECC

Total sector size
is measure of formatted capacity of disk as
fraction of nominal capacity
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Disk Miscellanea

• Smaller disks tend to be more cost-effective
• Smaller inertia, lower power consumption per
  megabyte, shorter seek distances, less vibration,
  less heat generated
• Heat generation proportional to N?RPM2.8 ?D4.6
• N number of platters
• D diameter
• RPM rotational speed
• Disk access time seek time rotational latency
  transfer time
• Typical values 12-30 ms seek time, 8.3 ms
  rotational latency
• Disk growth rates
• Disk areal density doubling every three years
• Disk transfer rate doubling every five years
• Disk access time halving every ten years

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Ensembles of Disks

• Key idea
• Use collection of disk drives to improve
  characteristics of disk systems (storage
  capacity, bandwidth, etc.)
• Used in mainframes for a long time
• RAID
• Redundant Array of Inexpensive Disks (original
  1988 acronym)
• Redundant Array of Independent Disks (redefined
  in 1992)

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Improving Bandwidth with Disk Arrays

• Arrays of independent disk drives
• Similar to high-order interleaving in main
  memories
• Each file assigned to different disk drive
• Simultaneous access to files
• Load balancing issues
• File striping/disk striping/disk interleaving
• Single file distributed across array of disks
• Similar to low-order interleaving in main
  memories
• Each logical I/O request corresponds to a data
  stripe
• Data stripe divided into number of equal sized
  stripe units
• Stripe units assigned to different disk units
• Two kinds of striping depending on size of stripe
  unit
• Fine-grained striping Stripe unit chosen to
  balance load
• Coarse-grained striping Larger stripe unit

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Improving Availability with Disk Arrays

• MTTF of large-system disks approaches 1,000,000
  hours
• MTTF of PC-class disks approaches 150,000 hours
• However, array of 1,000 PC-class disks has MTTF
  of 150 hours
• All schemes to cope with low MTTF aim to fail
  soft
• Operation should be able to continue while repair
  is made
• Always depends on some form of redundancy

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RAID-0

• Striped, non-redundant
• Parallel access to multiple disks
• Excellent data transfer rate (for small strips)
• Excellent I/O request processing rate (for large
  strips)
• Typically used for applications requiring high
  performance for non-critical data

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RAID-1

• Mirrored/replicated (most costly form of
  redundancy)
• I/O request rate good for reads, fair for writes
• Data transfer rate good for reads writes
  slightly slower
• Read can be serviced by the disk with the shorter
  seek distance
• Write must be handled by both disks
• Typically used in system drives and critical
  files
• Banking, insurance data
• Web (e-commerce) servers

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Combining RAID-0 and RAID-1

• Can combine RAID-0 and RAID-1
• Mirrored stripes (RAID 01, or RAID 01)
• Example picture above
• Striped Mirrors (RAID 10, or RAID 10)
• Data transfer rate good for reads and writes
• Reliability good
• Efficiency poor (100 overhead in terms of disk
  utilization)

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RAID-3

• Fine-grained (bit) interleaving with parity
• E.g., parity sum modulo 2 (XOR) of all bits
• Disks are synchronized, parity computed by disk
  controller
• When one disk fails (how do you know?)
• Data is recovered by subtracting all data in good
  disks from parity disk
• Recovering from failures takes longer than in
  mirroring, but failures are rare, so is okay
• Hot spares used to reduce vulnerability in
  reduced mode
• Performance
• Poor I/O request rate
• Excellent data transfer rate
• Typically used in large I/O request size
  applications, such as imaging or CAD

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RAID-2

• Hamming codes capable of correcting two or more
  erasures
• E.g., single error-correcting, double
  error-detecting (SEC-DED)
• Problem with small writes (similar to DRAM cycle
  time/access time)
• Poor I/O request rate
• Excellent data transfer rate

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RAID-4

• Coarse-grained striping with parity
• Unlike RAID-3, not all disks need to be read on
  each write
• New parity computed by computing difference
  between old and new data
• Drawback
• Like RAID-3, parity disk involved in every write
  serializes small reads
• I/O request rate excellent for reads, fair for
  writes
• Data transfer rate good for reads, fair for
  writes

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RAID-5

• Key Idea reduce load on parity disk
• Block-interleaved distributed parity
• Multiple writes can occur simultaneously
• Block 0 can be accessed in parallel with Block 5
• First needs disks 1 and 5 second needs disks 2
  and 4
• I/O request rate excellent for reads, good for
  writes
• Data transfer rate good for reads, good for
  writes
• Typically used for high request rate,
  read-intensive data lookup

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Removable Media

• Magnetic Tapes
• Have been used in computer systems as long as
  disks
• Similar magnetic technology as disks
• have followed similar density improvements as
  disks
• Long strips wound on removable spools
• Benefits can be essentially of unlimited
  length removable
• Drawbacks only offer sequential access wear
  out
• Used for archival backups but on the way out
• Optical Disks CDs and DVDs
• CD-ROM/DVD-ROM removable, inexpensive, random
  access
• CD-R/RW, DVD-R/RAM writable/re-writable
• Becoming immensely popular replacing floppy
  drives, tape drives
• Flash flash cards, memory sticks, digital film
• Microdrives small portable hard disks (up to
  1GB)

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Interfacing I/O Subsystem to CPU

• Where do we connect I/O devices?
• Cache?
• Memory? ?
• Low-cost systems I/O bus memory bus!
• CPU and I/O compete for bus
• How does CPU address I/O devs.?
• Memory-mapped I/O
• Portions of address space assigned to I/O devices
• I/O opcodes (not very popular)
• How does CPU synch with I/O?
• Polling vs. interrupt-driven
• Real-time systems hybrid
• Clock interrupts periodically
• CPU polls during interrupt

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Bus Different Options

• Bus width
• High-perf separate address and data lines
• Low-cost multiplex address and data lines
• Data width
• High-perf wider is faster (e.g., 64 bits)
• Low-cost narrower is cheaper (e.g., 8 bits)
• Transfer size
• High-perf multiple words have less bus overhead
• Low-cost single-word transfer is simpler
• Bus masters
• High-perf multiple (requires arbitration)
• Low-cost single master (simpler)
• Split transaction
• High-perf Yes (request and reply in separate
  packets)
• Low-cost No (continuous connection is cheaper)
• Clocking
• High-perf synchronous (though it is changing)
• Low-cost asynchronous

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Examples of Bus Standards
IDE/Ultra ATA SCSI PCI PCI-X
Data width 16 bits 8 or 16 bits 32 or 64 bits 32 or 64 bits
Clock rate Up to 100 MHz 10-160 MHz 33-66 MHz 66-133 MHz
Bus masters 1 Multiple Multiple Multiple
Peak Bandwidth 200 MB/sec 320 MB/sec 533 MB/sec 1066 MB/sec
Clocking Async Async Sync Sync
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Does I/O Performance Matter?

• The No Argument
• There is always another process to run while one
  process waits for I/O to complete
• Counter-arguments
• Above argument applies only if throughput is sole
  goal
• Interactive software and personal computers lay
  more emphasis on response time (latency), not
  throughput
• Interactive multimedia, transaction processing
• Process switching actually increases I/O (paging
  traffic)
• Desktop/Mobile computing only one
  person/computer ? fewer processes than in
  time-sharing
• Amdahls Law benefits of making CPU faster
  taper off if I/O becomes the bottleneck

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Does CPU Performance Matter?

• Consequences of Moores Law
• Large, fast CPUs
• Small, cheap CPUs
• Main goal keeping I/O devices busy, not keeping
  CPU busy
• bulk of the hardware cost is in I/O peripherals,
  not CPU
• Shift in focus
• From computation to communication
• Reflected in change of terminology
• 1960s-80s Computing Revolution
• 1990s-present Information Age

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Does Performance Matter?

• Performance is not the problem it once was!
• 15 years of doubling CPU speed every 18 months
  (x1000)
• Disks also have steadily become bigger and faster
• Most people would prefer more reliability than
  speed
• Todays speeds come at a cost frequent crashes
• Program crashes ? frustration
• Disk crashes ? hysteria
• Reliability, availability, dependability, etc.
  are the key terms
• Client-server model of computing has made
  reliability the key criterion for evaluation
• E.g., UNC-COMPSCI IMAP server 300 MHz or
  something, 99.9 uptime