External Memory

1
Chapter 6

• External Memory
• Disk and RAID (Redundant Arrays of Independent
  Disks)
• CS-147 Fall 2010
• Jonathan Wang

2
Magnetic Disk

• Coated with magnetizable material for read and
  write purpose.
• The substrat used to be aluminum.
• Recently use glass.
• Better stiffness
• Greater shock/damage resistance
• Lower fly height
• Improved uniformity of surface helps to reduced
  read-write errors

3
Magnetic Write and Read Mechanism

• Head
• Fixed head
• One read-write head per track
• Heads build into a fixed ridged arm
• Movable head
• One read-write head per surface
• Build into a movable arm

When the track passes under the head, it
generates a current of the same polarity as the
one already recorded.
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Disk Data Layout

• Contains
• Tracks
• Same with as the head.
• Intertrack Gaps
• Sectors
• Fixed-length (512 bytes) is commonly used in
  industry.
• InterSector Gaps

Gaps are there to minimize errors due to
misalignment of head or interference of magnetic
field.
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Disk Layout Methods

• CAV Constant Angular Velocity
• Multiple Zone Recording to enhance
  density(capacity)

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Characteristics

• Movable Head or not
• Removability
• Provides unlimited storage capacity
• Easy data transfer between systems
• Multiple Platter
• Single or double sided.

7
Disk Performance Parameters

• Seek Time time to position the head at the
  track
• Rotational Delay The time it takes for the
  begining of the sector to reach the head
• Transfer Time time required for the transfer
• T Transfer time
• b Number of bytes to be transfered
• N Number of bytes on a track
• r rotation speed in rev/sec
• Units usually is in ms, and considered average
  case

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RAID

• Stand for Redundant Arrays of Independent Disks
• RAID is a set of physical disk drives viewed by
  the perating system as a single logical drive
• Data are distributed across the physical drives
  of array in ascheme known as striping, describes
  subseuently.
• Redundant disk capacity is used to store parity
  information, which quarantees data recoverability
  in case of a disk failure.
• Uses Array Management Software
• Level 0 6 and more, such as RAID 10 (a
  combination of RAID 0 and RAID 1)

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RAID Level 0

• Not a true member of RAID family
• No redundancy or fault tolerance
• High transfer capacity for large and small I/O
  data
• It's there because it distrbites datas across
  mutiple disks
• No parity coculation is needed
• Easy to implement

10
RAID Level 0

• In a transaction environment, there may be
  hundreds of I/O requests per second. A disk array
  can provide high I/O executtion rates by
  balancing the I/O load across mutiple disks.
• Parallel processing
• Any error is uncorrectable
• One disk's failure will result in all data in an
  array being lost

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RAID Level 1

• Redundancy is achieved by having a mirror disk
• Insufficient use of space
• Read request is really efficiency (the one
  involves minimum seek time plus rotational
  latency)
• Write request could be done parallelly (T the
  larger one)
• Recovery is really simple.Just replace the
  broken diskwith a new one

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RAID

• Comparison see Table 6.4 p.204

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Reference

• Text Book - Computer Organization and
  Architecture Designing for Performance, 8th
  Edition By William Stallings, Prentice Hall
• Wikipedia http//en.wikipedia.org/wiki/RAID