Computer Peripherals

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Computer Peripherals Part I
2
Plan

• Storage (hierarchy and terminology)
• Magnetic disks

3
Storage

• Terminology
• Medium
• The technology or product type that holds the
  data
• Access time
• The time to get to the data
• Specified as an average in seconds (e.g., s, ms,
  µs, ns, etc.)
• Throughput
• The rate of transfer for consecutive bytes of
  data
• Specified in bytes/s (e.g., Kbytes/s, Mbytes/s)

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Storage Hierarchy
You should know this hierarchy
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Terminology

• Online storage
• Memory that is accessible to programs without
  human intervention
• Primary storage and secondary storage are
  online
• Primary storage
• Semiconductor technology (e.g., RAM)
• Volatile (contents might be lost when powered off
  )
• Secondary storage
• Magnetic technology (e.g., disk drives)
• Non-volatile (contents are retained in the
  absence of power)

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Terminology

• Offline storage
• Memory that requires human intervention in order
  for it to be accessed by a program (e.g., loading
  a tape)
• Sometimes called archival storage
• Direct Access Storage Device (DASD)
• Pronounced dazz-dee
• Term coined by IBM
• Distinguishes disks (disk head moves directly
  to the data) from tapes (tape reel must wind
  forward or backward to the data sequential
  access)

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Plan

• Storage (hierarchy and terminology)
• Magnetic disks

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Magnetic Disks

• A magnetic substance is coated on a round surface
• The magnetic substance can be polarized in one of
  two directions with an electromagnet (writing
  data)
• The electromagnet can also sense the direction of
  magnetic polarization (reading data)
• Similar to a read/write head on a tape recorder
  (except the information is digital rather than
  analogue)

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Hard Disks

• The platter is hard (e.g., aluminum)
• Most hard disk drives contain more than one
  platter
• On most hard disk drives, the disks are fixed
  (i.e., not removable)
• On some hard disk drives, the disks are in a
  removable pack (hence, disk pack)
• Typical speed of rotation 3600, 5400, 7200 rpm
  (rpm revolutions per minute)
• Capacities 500 MB to 10 TB (terabyte 240
  bytes)

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Hard Disk Layout
Head
Block
Headmotor
Platter
Sector
Track
1
220
2
Cylinder
3
Track
Drivemotor
Head, onmoving arm
Head assembly
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Hard Disk Example
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Terminology

• Platter
• A round surface the disk containing a
  magnetic coating
• Track
• A circle on the disk surface on which data are
  contained
• Head
• A transducer attached to an arm for
  writing/reading data to/from the disk surface
• Head assembly
• A mechanical unit holding the heads and arms
• All the head/arm units move together, via the
  head assembly
• Cylinder
• A set of tracks simultaneously accessible from
  the heads on the head assembly

YOU MUST KNOW THIS
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Terminology

• Drive motor
• The motor that rotates the platters
• Typically a DC motor (DC direct current)
• The disk rotates at a fixed speed (e.g., 3600
  rpm, revolutions per minute)
• Head motion
• A mechanism is required to move the head assembly
  in/out
• Two possibilities
• A stepper motor (digital, head moves in steps, no
  feedback)
• A servo motor (analogue, very precision
  positioning, but requires feedback)

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Terminology

• Sector
• That portion of a track falling along a
  predefined pie-shaped portion of the disk surface
• The number of bytes stored in a sector is the
  same, regardless of where the sector is located
  thus, the density of bits is greater for sectors
  near the centre of the disk
• The rotational speed is constant i.e., constant
  angular velocity
• Thus, the transfer rate is the same for inner
  sectors and outer sectors
• Block
• The smallest unit of data that can be written or
  read to/from the disk (typically 512 bytes)

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Locating a Block of Data
Seek Time
Latency Time
Transfer Rate
Latency
Transfer
Head
6
5
4
7
3
8
1
2
Seek
Desiredtrack
Note Access time seek time latency
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Terminology

• Seek time
• The time for the head to move to the correct
  track
• Specified as an average for all tracks on the
  disk surface
• Latency time
• The time for the correct block to arrive at the
  head once the head is positioned at the correct
  track
• Specified as an average, in other words, ½ the
  period of rotation
• Also called rotational delay
• Access time is the time to get to the data
  (remember!)
• Access time seek time latency
• Transfer rate
• Same as throughput

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Latency Example

• A hard disk rotates at 3600 rpm
• What is the average latency?

Period of rotation (1 / 3600)
?minutes (1 / 3600) ? 60
seconds 0.01667 s 16.67
ms Average latency 16.67 / 2 ms 8.33
ms
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Factors Determining Transfer Rate

• Transfer rate can be determined, given
• Rotational speed of the disk platters
• Number of sectors per track
• Number of bytes per sector

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Transfer Rate Example

• Q Determine the transfer rate, in Mbytes/s, for
  a hard disk drive, given
• Rotational speed 7200 rpm
• Sectors per track 30
• Data per sector 512 bytes 0.5 Kbytes
• A
• Transfer rate 7200 x 30 216,000 sectors/min
• 216,000 x 0.5 108,000 Kbytes/min
• 108,000 / 60 1,800 Kbytes/s
• 1,800 / 210 1.76 Mbytes/s

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Exercise - Transfer Rate

• Q Determine the transfer rate, in Mbytes/s, for
  a hard disk drive, given
• Rotational speed 7000 rpm
• Sectors per track 32
• Data per sector 1024 bytes

Skip answer
Answer
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• Q Determine the transfer rate, in Mbytes/s, for
  a hard disk drive, given
• Rotational speed 7000 rpm
• Sectors per track 32
• Data per sector 1024 bytes

A Transfer rate 7000 x 32 224,000
sectors/min 224,000 x 1 224,000
Kbytes/min 224,000 / 60 3,733 Kbytes/s
3,733 / 210 3.65 Mbytes/s
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Track Format

• Format of each track

data
header
gap
gap
CRC
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Figure 9.7 A single data block
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Figure 9.8 Header for MS-DOS/Windows disk
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Disk Formatting

• The track positions, blocks, headers, and gaps
  must be established before a disk can be used
• The process for doing this is called formatting
• The header, at the beginning of each sector,
  uniquely identifies the sector, e.g., by track
  number and sector number

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Disk Controller

• Interface between the disk drive and the system
  is known as a disk controller
• A primary function is to ensure data read/write
  operations are from/to the correct sector
• Since data rate to/from the disk is different
  than data rate to/from system memory, buffering
  is needed

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Buffering
Example Reading data from a disk
System
Diskcontroller
Disk
RAM
Buffer (RAM)
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Multi-block Transfers (1 of 2)

• The smallest transfer is one block (e.g., 512
  bytes)
• However, often multi-block transfers are required
• The inter-block gap provides time for the
  controller electronics to adjust from the end of
  one sector to the beginning of the next
• time may be needed for a few reasons
• Compute and/or verify the CRC bytes
• Switch circuits from read mode to write mode
• During a write operation the header is read but
  the data are written
• (Remember, the header is only written during
  formatting.)
• Perform a DMA operation

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Multi-block Transfers (2 of 2)

• Sometimes, sectors simply cannot be read or
  written consecutively
• There is not enough time (see preceding slide)
• The result is lost performance because the disk
  must undergo a full revolution to read the next
  sector
• The solution interleaving

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Interleaving
A must know item

• Rather than numbering blocks consecutively, the
  system skips one or more blocks in its numbering
• This allows multi-block transfers to occur as
  fast as possible
• Interleaving minimizes lost time due to latency
• Interleaving factor (see next slide) is
  established when the disk is formatted
• Can have a major impact on system performance

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Interleaving Examples
Factor
2
1
3
5
4
6
8
7
9
11
etc.
1
2
3
4
5
21
etc.
1
2
3
31
etc.
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21 Interleaving
2
6
1
7
5
3
9
8
4
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File System Considerations

• There is no direct relationship between the size
  and physical layout of blocks on a disk drive and
  the size and organization of files on a system
• File system
• Determines the organization of information on a
  computer
• Performs logical-to-physical mapping of
  information
• A file system is part of each and every operating
  system
• Logical mapping
• The way information is perceived to be stored
• Physical mapping
• The way information is actually stored

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Alternative Technologies (1 of 3)

• Removable hard disks
• Also called disk packs
• A stack of hard disks enclosed in a metal or
  plastic removable cartridge
• Advantages
• High capacity and fast, like hard disk drives
• Portable, like floppy disks
• Disadvantage
• Expensive

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Alternative Technologies (2 of 3)

• Fixed heads
• Fewer tracks but eliminates seek time

Moving head
Disk
Spindle
Fixed heads
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Alternative Technologies (3 of 3)

• R.A.I.D. Redundant array of inexpensive disks
• A category of disk drive that employs two or more
  drives in combination for fault tolerance and
  performance
• Frequently used on servers, but not generally
  used on PCs
• There are a number of different R.A.I.D. levels
  (next slide)

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R.A.I.D. Levels (1 of 2)

• Level 0
• Provides data striping (spreading out blocks of
  each file across multiple disks)
• No redundancy
• Improves performance, but does not deliver fault
  tolerance
• Level 1
• Provides data mirroring (a.k.a. shadowing)
• Data are written to two duplicate disks
  simultaneously
• If one drive fails, the system can switch to the
  other without loss of data or service
• Delivers fault tolerance

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R.A.I.D. Levels (2 of 2)

• Level 3
• Same as level 0, but also reserves one dedicated
  disk for error correction data
• Good performance, and some level of fault
  tolerance
• Level 5
• Data striping at the byte level and stripe error
  correction information
• Excellent performance, good fault tolerance

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Terminology

• Fault tolerance
• The ability of a computer system to respond
  gracefully to unexpected hardware or software
  failure
• Many levels of fault tolerance
• E.g., the ability to continue operating in the
  event of a power failure
• Some systems mirror all operations
• Every operation is performed on two or more
  duplicate systems, so if one fails, another can
  take over

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Terminology

• Data mirroring (also shadowing)
• A technique in which data are written to two
  duplicate disks simultaneously
• If one disk fails, the system can instantly
  switch to the other disk without loss of data or
  service
• Used commonly in on-line database systems where
  it is critical that data are accessible at all
  times

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Terminology

• Data striping
• A technique for spreading data over multiple
  disks
• Speeds operations that retrieve data from disk
  storage
• Data are broken into units (blocks) and these are
  spread across the available disks
• Implementations allow selection of data units
  size, or stripe width

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CD-ROM

• CD-ROM stands for compact disc, read-only
  memory
• Evolved from audio CDs
• Disk size 120 mm (5¼)
• Capacity 750 MB

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Operation

• Uses light generated by lasers to record and
  retrieve information
• Information is stored by varying the light
  reflectance characteristics of the medium
• Available in read-only (CD-ROM) and read/write
  formats

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Layout of a CD-ROM versus a standard disk
75 sectors/second 60 sec/min
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CD-ROM vs. Magnetic Disk
CD-ROM Magnetic Disk
One spiral track (3 miles long!) Multiple tracks of concentric circles
Constant bit density Variable bit density
Disk speed varies (CLV, constant linear velocity) Disk speed constant (CAV, constant angular velocity)
Constant transfer rate Constant transfer rate
Capacity 750 MB Capacity varies
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CD-ROM Data Organization

• 270,000 blocks of 2048 bytes each (typically)
• 270,000 ? 2048 552,960,000 bytes
• Extensive error checking and correction (e.g.,
  bad regions of the disk flagged)
• Substantial overhead for error correction and
  identifying blocks
• Capacity can be as high as 750 MB

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Pits and Lands (1 of 2)

• Data are stored as pits and lands
• These are burned into a master disk by a high
  powered laser
• Master disk is reproduced mechanically by a
  stamping process. ( Like a coin, sort of. )
• Data surface is protected by a clear coating
• Data are read by sensing the reflection of laser
  light
• A pit scatters the light
• A land reflects the light

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Pits and Lands (2 of 2)
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CD-ROM Read Process
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Magneto Optical

• Disk may be written, read, and rewritten
• Write process is preformed at high temperature
• Combines features of optical and magnetic
  technology
• Data are stored as a magnetic charge on the disk
  surface
• During reading, the polarity of the reflected
  light is sensed (not the intensity)

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• http//en.wikipedia.org/wiki/DVD-R