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Secondary or External Computer Memory

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					EXTERNAL MEMORY

Types of External Memory

    Magnetic Disk

            RAID

            Removable

    Optical

            CD-ROM

            CD-Writable (WORM)

            CD-R/W

            DVD

    Magnetic Tape

Magnetic Disks

    The disk is a metal or plastic platter coated with magnetically material

    Data is recorded onto and later read from the disk using a conducting coil, the head

    Data is organized into concentric rings, called tracks, on the platter

    Tracks are separated by gaps

    Logical data transfer unit is the sector

            Sectors are identified on each track during the formatting process

    Disk rotates at a constant speed– constant angular velocity

            Number of data bits per track is a constant

            Data density is higher on the inner tracks

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

     Minimum block size is one sector

     May have more than one sector per block

Disk characteristics

     Single vs. multiple platters per drive (each platter has its own read/write head)

     Fixed vs. movable head

             Fixed head has a head per track

             Movable head uses one head per platter

     Removable vs. no removable platters

             Removable platter can be removed from disk drive for storage of transfer to another
              machine

Data accessing times

     Seek time– position the head over the correct track

     Rotational latency– wait for the desired sector to come under the head

     Access time– seek time plus rotational latency

     Block transfer time– time to read the block (sector) off of the disk and transfer it to main
      memory

RAID Technology

     Disk drive performance has not kept pace with improvements in other parts of the system

     Limited in many cases by the electro mechanical transport means

     Capacity of a high performance disk drive can be duplicated by operating many (much cheaper)
      disks in parallel with simultaneous access

     Data is distributed across all disks

     With many parallel disks operating as if they were a single unit, redundancy techniques can be
      used to guard against data loss in the unit (due to aggregate failure rate being higher)
    RAID developed at Berkeley– Redundant Array of Independent Disks

            Six levels:0--5

RAID 0

    No redundancy techniques are used

    Data is distributed over all disks in the array

    Data is divided into strips for actual storage

            Similar in operation to interleaved memory data storage

    Can be used to support high data transfer rates by having block transfer size be in multiple of
     the strip

    Can support low response time by having the block transfer size equal a strip—support multiple
     strip transfers in parallel

RAID 1

    All disks are mirrored—duplicated

            Data is stored on a disk and its mirror

            Read from either the disk or its mirror

            Write must be done to both the disk and mirror

    Fault recovery is easy----use the data on the mirror

    System is expensive

RAID 2

    All disks are used for every access—disks are synchronized together

    Data strips are small (byte)

    Error correcting code computed across all disks and stored on additional disks

    Uses fewer disks than RAID 1 but still expensive

            Number of additional disks is proportional to log of number of data disks

RAID 3

    Like RAID 2 however only a single redundant disk is used---the parity drive
     Parity bit is computed for the set of individual bits in the same position on all disks

     If a drive fails, parity information on the redundant disks can be used to calculate the data from
      the failed disk “on the fly”

RAID 4

     Access is to individual strips rather than to all disks at once (RAID 3)

     Bit-by-bit parity is calculated across corresponding strips on each disk

     Parity bits stored in the redundant disk

     Write penalty

             For every write to a strip, the parity strip must also be recalculated and written

             Thus 1 logical write equals 2 physical disk accesses

             The parity drive is always written to and can thus be a bottleneck

RAID 5

     Parity information is distributed on data disks in a round- robin scheme

     No parity disk needed

Optical Disks

     Advent of CDs in the early 1980s revolutionized the audio and computer industries

     Basic operation

             CD is operated using constant linear velocity

             Essentially one long track spiraled on to the disk

             Track passes under the disk’s head at a constant rate– requires the disk to change
              rotational speed based on what part of the track you are on

             To write to the disk, a laser is used to burn pits into the track—write once

             During reads, a low power laser illuminates the track and its pits

                     In the track, pits reflect light differently than no pits thus allowing you to store
                      1s and 0s

             Master disk is made using the laser

                     Master is used to “press” copies in a mass production mechanical style
                     Cheaper than production of information on magnetic disks

     Storage capacity roughly 775NB or 559 3.5” disks

     Transfer standard is 176 MB/second

     Only economical for production of large quantities of disks'

     Disks are removable and thus archival

     Slower than magnetic disks

WORMs

     WORMs– Write once, Read many disks

     User can produce CD ROMs in limited quantities

     Specially prepared disk is written to using a medium power laser

     Can be read many times just like a normal CD ROM

     Permits archival storage of user information, distribution of large amounts of information by a
      user

Erasable optical Disk

     Combines laser and magnetic technology to permit information storage

     Laser heats an area that can then have an e-field orientation changed to alter information
      storage

     “state of the e-field” can be detected using polarized light during reads

DVD - what’s in a name?

     Digital Video Disk

             Used to indicate a player for movies

                     Only plays video disks

     Digital Versatile Disk

             Used to indicate a computer drive

                     Will read computer disks and play video disks

     Dogs Veritable Dinner
    Officially - nothing!!!

DVD – technology

    Multi-layer

    Very high capacity (4.7G per layer)

    Full length movie on single disk

            Using MPEG compression

    Finally standardized (honest!)

    Movies carry regional coding

    Players only play correct region films

    Can be “fixed”

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!

Magnetic Tape

    The first kind of secondary memory

    Still widely used

            Very cheap

            Very slow

    Sequential access

            Data is organized as records with physical air gaps between records

            One words is stored across the width of the tape and read using multiple read/write
             heads

Digital Audio Tape (DAT)

    Uses rotating head (like video)
 High capacity on small tape

        4Gbyte uncompressed

        8Gbyte compressed

 Backup of PC/network servers

				
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Description: Basics knowledge about computer external memory