CSCI 3100, Computer Organization and Architecture

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CSCI 3100, Computer Organization and
Architecture

• Chapter 6 External Memory
• Instructor Ahmad Ghafarian

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Outline

• Magnetic disk
• Fixed, i.e. hard drive
• Removable, i.e. floppy, Zip drive, Superdisks,
  tape
• Parallel array of storages called RAID
• Optical disk
• CD ROMCD Recoverable
• CD read/write
• DVD

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Magnetic Storage

• Exploits duality of magnetism and electricity
• Converts electrical signals into magnetic charges
• Captures magnetic charge on a storage medium
• Later regenerates electrical current from stored
  magnetic charge
• Polarity of magnetic charge represents bit values
  zero and one

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Magnetic Disk

• Disk constructed of substrate coated with
  magnetizable material (iron oxiderust)
• Substrate used to be aluminium or aluminium alloy
  material
• Now glass substrate is used with the following
  features
• Improved surface uniformity
• Increases reliability
• Reduction in surface defects
• Reduced read/write errors
• Lower flight heights (See later)
• Better stiffness
• Better shock/damage resistance

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Read and Write Mechanisms

• Recording retrieval via conductive coil called
  a head
• May be single read/write head or separate ones
• During read/write, head is stationary, platter
  rotates
• Write
• Current through coil produces magnetic field
• Pulses sent to head
• Magnetic pattern recorded on surface below
• Read (traditional)
• Magnetic field moving relative to coil produces
  current
• Coil is the same for read and write
• Read (contemporary)
• Separate read head, close to write head
• Partially shielded magnetoresistive (MR) sensor
• Electrical resistance depends on direction of
  magnetic field
• High frequency operation
• Higher storage density and speed

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Read/write Summary

• Flat, circular platter with metallic coating that
  is rotated beneath read/write heads
• Random access device read/write head can be
  moved to any location on the platter
• Hard disks and floppy disks
• Cost performance leader for general-purposeon-lin
  e secondary storage

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Inductive Write MR Read
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Data Organization and Formatting

• Concentric rings or tracks
• Gaps between tracks
• Reduce gap to increase capacity
• Same number of bits per track (variable packing
  density)
• Constant angular velocity
• Tracks divided into sectors (typical 10-100
  sector per track) 512 byte size of each sector
• Minimum block size is one sector
• May have more than one sector per block

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Disk Data Layout
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Constant Angular Velocity CAV

• Gives pie shaped sectors and concentric tracks
• Individual tracks and sectors addressable
• Move head to given track and wait for given
  sector
• Waste of space on outer tracks
• Lower data density (because inner and out tracks
  sectors can hold the same amount of information
• Instead we can use Multiple Zoned recording

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Disk Layout Methods Diagram
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Multiple Zone Recording (1)

• tracks are grouped into zones
• based on their distance from the center of the
  disk
• each zone is assigned a number of sectors per
  track
• As you move from the innermost part of the disk
  to the outer edge
• you move through different zones
• each containing more sectors per track than the
  one before.
• This allows for more efficient use of the larger
  tracks on the outside of the disk.

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Multiple Zone Recording (2)

• This model hard diskhas 20 tracks.
• They have been divided into five zones
• The blue zone has 5 tracks, each with 16 sectors
• the cyan zone 5 tracks of 14 sectors each
• the green zone 4 tracks of 12 sectors
• the yellow 3 tracks of 11 sectors
• the red 3 tracks of 9 sectors.
• the size (length) of a sector remains fairly
  constant over the entire surface of the disk
• If not for MZR, every track on this hard disk
  would be limited to only 9 sectors, greatly
  reducing capacity.

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Finding Sectors

• Must be able to identify start of track and
  sector
• Handled by control data recorded on disk
• Additional information are also stored that are
  not available to user

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Winchester Disk Format Seagate ST506
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Physical Characteristics

• Head Motion
• Fixed head (rare) one per track
• Moveable, one per surface
• Disk portability
• Nonremovable disk, e.g. hard drive in PCs
• Removable disk, e.g. floppy, ZIP
• Sides
• Single sided
• double (usually) sided
• Platter
• Single
• Multiple figure 6.5
• Head mechanism
• Contact (Floppy)
• Fixed gap, greater rsisk of error
• Flying (Winchester)

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Fixed/Movable Head Disk

• Fixed head
• One read write head per track
• Heads mounted on fixed ridged arm
• Movable head
• One read write head per side
• Mounted on a movable arm

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Removable or Not

• Removable disk
• Can be removed from drive and replaced with
  another disk
• Provides unlimited storage capacity
• Easy data transfer between systems
• Nonremovable disk
• Permanently mounted in the drive

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Multiple Platter

• One head per side
• Heads are joined and aligned
• Aligned tracks on each platter form cylinders
• Data is striped by cylinder
• reduces head movement
• Increases speed (transfer rate)

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Multiple Platters
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Tracks and Cylinders
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Floppy Disk

• 8, 5.25, 3.5
• Small capacity
• Up to 1.44Mbyte (2.88M never popular)
• Slow
• Universal
• Cheap
• Obsolete?

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Winchester Hard Disk (1)

• Developed by IBM in Winchester (USA)
• Sealed unit
• One or more platters (disks)
• Heads fly on boundary layer of air as disk spins
• Very small head to disk gap
• Getting more robust

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Winchester Hard Disk (2)

• Universal
• Cheap
• Fastest external storage
• Getting larger all the time
• 250 Gigabyte now easily available

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Speed

• Seek time
• Moving head to correct track
• (Rotational) latency
• Waiting for data to rotate under head
• The time for reading data consist
• Access time Seek Latency
• Transfer rate n the time required to actually
  read/write data
• Queuing time wait time for the device to be
  available

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Timing of Disk I/O Transfer
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RAID (1)

• Redundant Array of Independent (or Inexpensive)
  Disks (RAID)
• 7 levels in common use (level 0-6)
• Different levels let us choose between
  performance, protection, and storage capacity.
• Not a hierarchy
• Set of physical disks viewed as single logical
  drive by O/S
• Data distributed across physical drives
• Can use redundant capacity to store parity
  information

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RAID (2)

• Each level is optimized for various capabilities,
  including
• improved performance of read or write operations,
  and
• improved data availability through redundant
  copies or parity checking.
• Features of different RAID levels can be combined
  to get the benefits of both.

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RAID Level 0

• high-performance/low-availability level.
• provides basic disk striping (across each disk)
  for improved performance
• no parity protection to catch errors
• no redundancy is provided.
• relatively inexpensive.
• If one disk in the array happens to fail, all
  data in the array is unavailable.

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Striping

• Striping is the practice of spreading data over
  multiple disk drives.
• It allows greater performance because drives can
  seek and deliver data simultaneously

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RAID level 1

• disk-mirroring strategy for high performance.
• all data is written twice to separate drives.
• The cost per megabyte of storage is higher, of
  course, but if one drive fails, normal operations
  can continue with the duplicate data.
• If the RAID device permits hot-swapping of
  drives, the bad drive can be replaced without
  interruption.

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RAID level 2

• performs disk striping at the bit level
• uses one or more disks to store parity
  information
• RAID-2 is not used very often because
• It is slow and
• Expensive
• Bit interleave data striping with hamming code.
• Fast for sequential applications such as graphics
  modeling
• Almost never used with PC-based systems

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RAID level 3

• uses data striping at the byte level which
• provides excellent performance when transferring
  large sequential amounts of data, because all
  disks operate in parallel
• uses one disk to store parity information which
• reduces the cost per megabyte of storage
• Two disks must fail before data would become
  unavailable
• Poor for random transactions
• Faster than a single disk but significantly
  slower than RAID 0 or RAID 1 in random
  environments

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RAID 4

• Each disk operates independently
• Good for high I/O request rate
• Stripping large chunk of data
• Bit by bit parity calculated across stripes on
  each disk
• slower than RAID 0 or RAID 1.

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RAID 5

• stripes data in blocks sequentially across all
  disks in an array
• writes parity data on all disks as well
• by distributing parity information across all
  disks, RAID-5 eliminates the bottleneck sometimes
  created by a single parity disk
• RAID-5 is increasingly popular and is well suited
  to transaction environments.
• usually used on servers

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RAID 6

• Two parity calculations
• Stored in separate blocks on different disks
• High data availability
• Three disks need to fail for data loss

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RAID 0, 1, 2
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RAID 3 4
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RAID 5 6
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Data Mapping For RAID 0
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Optical Storage CD-ROM

• Originally for audio
• 650 Mbytes giving over 70 minutes audio
• Polycarbonate coated with highly reflective coat,
  usually aluminium
• Data stored as pits
• Read by reflecting laser
• Constant packing density
• Constant linear velocity

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CD Operation
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CD-ROM Drive Speeds

• Generally stated in terms of multiples of
  standard
• originally was 150KB/second
• newer CDs can have up to 72x (10800 KB/second),
  means multiples of original speed
• average seek time 200 ms to 80 ms
• current hard drives is 9ms
• CD-ROM drives usually operate using Constant
  Linear Velocity (CLV).

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CD-ROM Format

• the header contains the block address mode byte
• Mode 0blank data field
• Mode 12048 byte dataerror correction
• Mode 22336 byte data

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Random Access on CD-ROM

• Difficult
• Move head to rough position
• Set correct speed
• Read address
• Adjust to required location

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CD-ROM pros cons

• Large capacity
• Easy to mass produce
• Removable
• Robust
• Expensive for small runs
• Slow
• Read only

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Other Optical Storage

• CD-Recordable (CD-R)
• it is known as write once and read more
• it uses different technology than CD-ROM, i.e.
  dye layer
• CD-RW
• it is known as CD writeable, van be repeatedly
  written overwritten
• Erasable
• Getting cheaper
• Mostly CD-ROM drive compatible
• Phase change
• Material has two different reflectivities in
  different phase states

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DVD - whats in a name?

• DVD is a form of storage media based on the CD
• Originally called Digital Video Disc
• later Digital Versatile Disc
• Essentially, DVD is a larger, faster CD that can
  hold video, audio, and/or computer data
• Physically similar to a CD, a single-layer,
  single-sided DVD has a maximum capacity of 4.7GB
  (about two hours of MPEG-2 video), about seven
  times the capacity of a CD-ROM (a normal CD holds
  around 650MB)
• A double-layer, double-sided DVD-ROM disk has
  thirty times the capacity of a CD-ROM (over
  17GB). The DVD specification supports access
  rates of 600KBps to 1.3MBps.
• DVD-ROM is DVD when used as computer storage.
  DVD-R is a once-recordable form of DVD, which
  requires a special drive and media, while such
  formats as DVD-RAM, DVD-RW, and DVD-R/RW can be
  written multiple times.

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DVD - technology

• Designed from the outset for video, audio and
  multimedia. Meets the requirement for 133 minutes
  of high quality video on one side of a disk.
• DVD-ROM for enhanced multimedia and games
  applications.
• DVD-Video for full length high quality movies on
  one disc.
• DVD-Audio for higher quality music, surround
  sound and optional video, graphics and other
  features.
• All formats use a common file system.
• Copy protection built into standard (unless it is
  broken...)

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DVD Writable

• Loads of trouble with standards
• First generation DVD drives may not read first
  generation DVD-W disks
• First generation DVD drives may not read CD-RW
  disks
• Wait for it to settle down before buying!

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CD and DVD
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Magnetic Tape

• Serial access
• Slow
• Very cheap
• Backup and archive

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References

• Optical Storage Technology Association
• Good source of information about optical storage
  technology and vendors
• Extensive list of relevant links
• DLTtape
• Good collection of technical information and
  links to vendors
• Search on RAID
• http//www.pcguide.com/ref/hdd/geom/tracksZBR-c.ht
  ml
• http//www.mckinnonsc.vic.edu.au/la/it/ipmnotes/se
  curity/RAID/raid.htm