The Hard Disk Secondary Storage

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The Hard DiskSecondary Storage

• Stewart Blakeway,
• FML208,
• blakews_at_hope.ac.uk
• 0151 291 3113 
• http//hopelive.hope.ac.uk/computing/

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This Week

• How Secondary Storage works
• ie the Hard Disk

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Hard Disk Drive

• HDD
• FIXED DISK DRIVE
• C DRIVE
• DRIVE C
• Winchester (very old term)

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Hard Disks

• They are most commonly known as Hard Disks
• This was to distinguish them from Floppy Disks
• Hard disks have a hard platter that holds the
  magnetic medium
• Floppy Disks use flexible plastic film

Whatever you call it. It is still secondary
storage
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Question

• If a Hard Disk is secondary storage, what is
  primary storage?

Answer RAM
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Characteristics of the Hard Disk

• The hard disk is NON-VOLATILE
• The hard disk is HUGE
• The hard disk is CHEAP
• So what is the downside to hard drives?

Answer They are very slow
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The Magnetic Disk

• The Magnetic Disk spins very quickly

• The disk spins are measured in rpm. Revolutions
  per Minute
• One full spin or cycle is 1 revolution

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Question

• How many revolutions per minute can modern hard
  disks achieve?
• 1 spin per second (60 rpm)
• 2 spins per second (120 rpm)
• 3 spins per second (180 rpm)
• 120 spins per second (7200 rpm)

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Answer

• How many revolutions per minute can modern hard
  disks achieve?
• D) 120 spins per second (7200 rpm)
• This might seem very fast. But its still slower
  than RAM
• The faster workstation or server can expect
  speeds of up to 15,000 rpm

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Performance

• Data rate - The data rate is the number of bytes
  per second that the drive can deliver to the RAM.
  Rates between 5 and 40 Megabytes per second are
  common.
• Access time - The access time is the amount of
  time between when the CPU requests a file and
  when the first byte of the file is sent to the
  RAM. Times between 10 and 20 milliseconds are
  common. The access time contains 4 factors

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Access Time

• Command Overhead Time
• The time that elapses from when a command is
  given to the hard disk until something actually
  starts happening to fulfil the command. In a way,
  it's sort of like a "reaction time" for the disk.
  
• Seek Time
• The seek time of a hard disk measures the amount
  of time required for the read/write heads to move
  between tracks over the surfaces of the platters.

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Access Time Continued

• Settle Time
• This refers to the amount of time required, after
  the head assembly has moved during a seek, for
  the heads to stabilize sufficiently for the data
  to begin to be read
• Latency
• The spin speed is not synchronized to the process
  that moves the read/write heads to the correct
  Track. Latency is the time between arriving at
  the correct track and the time locating the
  correct sector

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Access Time Continued

• Seek Time and Latency are the biggest overheads
  in relation to transferring data, especially
  because files are usually split over separate
  sectors
• Command Overhead Time and Settle Time overheads
  are small in contrast

Generally speaking the faster the rpms the
faster the data rate and access time. This can
however vary between manufacturers
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Capacity

• A Floppy Disk typically holds 1.44 Megabytes of
  data
• A pen drive typically holds
• 64Megabytes
• 128Megabytes
• 256Megabytes
• 512Megabytes
• 1Gigabyte (1024Megabytes)

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Capacity

• The first Hard Disk was commercially available in
  1956 and stored approximately 5Megabytes - This
  is small, but this was amazing 50 years ago
• In the early 1980s the average capacity of a
  Hard Disk was 20Megabytes
• The Mid 90s Hard Disks of 1Gigabytes became
  common
• The end of the 1990s 20Gigabytes was common

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Question

• What was the largest capacity Hard Disk that can
  be purchased?
• 40 Gigabytes
• 60 Gigabytes
• 100 Gigabytes
• 750 Gigabytes

750 Gigabytes is equivalent to 768,000 Megabytes
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Physical Size

• Although Capacity has increased
• The physical size has decreased

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Platters

• There are usually more than one rotating disks.
  These are called platters

• Each platter has two sides. Top and Bottom

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Platters

• Non-magnetic material
• Usually glass or aluminium
• Coated with a thin layer of magnetic material

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Heads

• Heads are required to write and read data to a
  platter
• The head is attached to an arm that is controlled
  by a motor
• The head moves across the platter as instructed
  by the Operating System

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Platters

• The storage capacity of the hard drive is
  increased by having more than one platter?

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Close up of a Head
Notice the small wires at the end of the head.
These connect to the Drive Electronics Board.
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The Head

• The hard disk is slower than RAM because RAM is
  purely electronic
• The hard disk has moving mechanical parts that
  slows the process of transferring data

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Question

• If a Hard Disk contains 3 Platters how many Heads
  will it have?
• Answer is 6. Each platter has two sides.

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Characteristics of the Arm

• The arm is able to move the heads from the hub to
  the edge of the drive.
• The arm and its movement mechanism are extremely
  light and fast.
• The arm on a typical hard-disk drive can move
  from the hub to the edge and back up to 50 times
  per second
• The Arm and Head never touch the platter. They
  hover just over it on a cushion of air

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Motor

• We have seen that a motor is required to move the
  head in and out across the platter.
• The Hard Disk uses more than one Motor to do
  this.

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Question

• Pen Drives do not have moving parts. They too
  are purely electronic. Does this mean they work
  as fast as RAM?
• YES
• NO

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Question

• Pen Drives do not have moving parts. They too
  are purely electronic. Does this mean they work
  as fast as RAM?
• Answer is NO. Pen Drives are Serial USB devices.
  Thereby restricted to the speed of the USB port
  transfers data at a maximum of 480 Mbit/s

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What controls the Head?

• Hard Disks need circuitry to control the head.
• These are separate from the Hard Disk

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IDE / EIDE

• The circuitry although separate from the Drive is
  never purchased separately.
• Because it comes as part of the Drive it is known
  as Integrated Drive Electronics
• Early computers Drive Electronics were part of
  the motherboard or an expansion card
• IDE became obsolete and was replaced by EIDE
  (Enhanced Integrated Drive Electronics)

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Today

• EIDE has been superseded by ATA (Advanced
  Technology Attachment)
• This is the most common interface available
  today. However this interface is still known most
  commonly by its original name IDE.
• This is rapidly becoming less common with the
  introduction of SATA (Serial ATA)

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Do computers need a Hard Disk?

• Early computer did not include hard disk. The
  Amiga, Sinclair Spectrum, Commodore are examples
  of computers that used only Floppy Diskette
  Drives or Tape for storage
• PCs sometimes booted up from EPROM (the network
  card) and did not use their own hard disk.
  Workstations connected to a mainframe are an
  example of this.
• Today it is very rare that you would purchase or
  use a computer without a hard disk. The entire
  operating System would come on a Single Floppy
  Disk.
• Today Linux is one of the Few Operating Systems
  that contain the entire Operating System on CD
  and does not require installing to the Hard Disk.

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Recording or Reading Data

• The data is recorded by magnetising the magnetic
  material in a pattern.
• The pattern represents data.
• They read the data back by detecting the
  magnetisation of the material.

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Organisation of the Hard Disk

• Before Data can be stored on the Hard Disk the
  Hard Disk needs to be split and organised into
  small magnetic domains.
• Data is stored in these small magnetic domains.
• Often Data is too big for a domain so it is split
  across several domains.

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How is the Hard Disk Split?

• The Hard Disk is split into Sectors and Tracks
• Tracks are Concentric Circles
• Sectors are pie shaped wedges within a track

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How is the Hard Disk Split?

• Each Sector contains a fixed number of bytes
• Sectors are often grouped together and are called
  Clusters

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How are the Tracks and Sectors defined?

• The process of Low-Level Formatting a drive
  establishes the tracks and sectors on the
  platter. The starting and ending points of each
  sector are written onto the platter.
• Low-level Formatting is done by the Manufacture
  of the drive
• This process prepares the drive to hold blocks of
  bytes

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High Level Formatting

• High-level formatting then writes the
  file-storage structures
• The File Allocation Tables
• This process enables the drive to store files,
  and importantly to locate them once they are
  stored.
• The Operating System being used dictates the
  Structure and File Allocation Tables being used

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What have we covered?

• Characteristics of the Hard Disk
• The Magnetic Disk
• Performance
• Capacity
• Physical Size
• Platters
• Heads
• Characteristics of the Arm

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What have we covered?

• Integrated Drive Electronics
• Recording or Reading Data
• Organisation of the Hard Disk
• How the Hard Disk is split
• How the Tracks and Sectors are defined
• Low Level Formatting
• High Level Formatting