Computer Component : Storage Device External Memory, Secondary Memory, Secondary Storage

1
Computer Component Storage Device(External
Memory, Secondary Memory, Secondary Storage)

• Storage Types
• Magnetic Types
• Optical Types

2
Storage Types

• Shape Types
• Disk (Fixed, Removable)
• Tape (Cartridges, Cassette)

• Technology Types
• Magnetic (Disk Tape)
• Optical (Disk)
• Access Method Types
• Sequential Access (Magnetic Type)
• Direct Access (Magnetic Optical Disk)

3
Magnetic Types

• Magnetic Disk
• Definition
• Data Organization, Formatting and Disk Layout
  Method
• Physical Characteristic
• Disk Performance Parameters
• Magnetic Tape
• Definition
• Data Organization and Formatting

4
Magnetic Disk Definition

• A Disk
• Circular platter constructed of metal or of
  plastic coated with a magnetizable material.
• Data are recorded on and retrieved from the disk
  via a conducting coil, head.
• To write, electricity flows through a coil
  produces a magnetic field then magnetic patterns
  (positive or negative) are recorded.
• To read, magnetic field moves relative to a coil
  produces an electrical current of the same
  polarity as it recorded in the coil

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Magnetic Disk Data Organization

• Tracks - the organization of data on the platter
  in a concentric set of rings, each track is the
  same width as the head.
• Data are transferred to and from a disk in blocks
• Sectors - data are stored in these block-size
  regions that maybe either fixed or variable
  length.
• Adjacent tracks or sectors are separated by gaps
• Density - bits per inch, increases from outer
  track to inner track
• Clusters - groups of sectors that use to store a
  file
• Cylinders - tracks in the same position of each
  side in multiple platter

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Magnetic Disk Formatting

• A process that prepare a disk for use
• Create tracks and sectors on disk
• Record information of disk
• number of byte/sector and sector/track
• File Allocation Table (FAT)
• Root folder (directory)
• A bit near the center of a rotating disk travels
  past a fixed point (e.g.. read-write head)
  slower than a bit on the outside.

7
Magnetic Disk Disk Layout Methods

• Multiple Zone Recording
• devide disk into a number of zones (16)
• within a zone, the number of bits per track is
  constant
• zones farther from the center contain more bits
  (more sectors) than the closer.
• the time to move a head for reads and writes vary
  from one zone to another.
• Pro increasing capacity

• Constant Angular Velocity (CAV)
• increasing space between bits
• bits can be scanned at the same rate
• devide disk into a number of pie-shaped sectors
  and a series of concentric tracks
• Pro block can be directly address by track and
  sector
• Con long outer track can store data in the same
  amount as short inner tracks

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Magnetic Disk Physical Characteristics

• Head Motion
• Fixed head (one per track)
• Movable head (one per surface)
• Disk Portability
• Nonremovable disk
• Removable
• Sides
• Single sided
• Double sided

• Platters
• Single platter
• Multiple platters
• Head Mechanisms
• Contact (Floppy)
• Fixed gap
• Aerodynamic gap (Winchester)

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Magnetic Disk Disk Performance Parameters

• Average Access Time ( Ta )
• Seek time ( Ts )
• Latency time ( r - Rotational delay )
• Transfer time ( T )

Ta Ts 1 T 2r
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Magnetic Disk Disk Performance Parameters

• Seek Time ( Ts )
• The time required to move the head to the
  required track
• A movable-head system moves the head
• A fixed-head system electrically selects one head
• m constant depends on the disk drive
• n number of tracks traversed
• s startup time

Ts m x n s
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Magnetic Disk Disk Performance Parameters

• Latency Time ( r - Rotational delay )
• The time required to move the beginning of the
  sector to the head
• Floppy disk rotates 300-600 rpm, average delay
  will be 100-200 ms
• Other disks rotate 3600-15000 rpm (1 revolution
  per 16.7 ms to 1 revolution per 4 ms ), average
  delay will be 8.3 ms

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Magnetic Disk Disk Performance Parameters

• Transfer Time ( T )
• The time to transfer data to or from the disk
• Depend on the rotation speed of the disk
• b number of bytes to be transferred
• N number of bytes on a track
• r rotation speed, in revolutions per second

T b rN
13
Magnetic Disk A Timing Comparison
Ts 20 ms, transfer rate 1 MB/s, 512
bytes/sector, 32 sectors/track, a file consists
of 256 sectors for 128 KB

• A sequential organization
• (8 adjacent tracks x 32 sector)
• Average seek 20.0 ms
• Rotational delay 8.3 ms
• Read 32 sectors 16.7 ms
• 45 ms
• Read 1 track 8.316.7 25 ms
• Ta 45 7 x 25 220 ms

• A random organization
• Average seek 20.0 ms
• Rotational delay 8.3 ms
• Read 1 sector 0.5 ms
• 28.8 ms
• Ta 256 x 28.8 7373 ms

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Magnetic Tape Definition

• The medium is flexible polyester tape coated with
  magnetizable material.
• The tape and tape drive are analogous to a home
  tape recorder system.
• Use the same reading and recording techniques as
  disk system.
• Tapes used to be packaged as open reels that have
  to be threaded through a second spindle for use.
• Today all tapes are housed in cartridge

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Magnetic Tape Data Organization Formatting

• Structured as a number of parallel tracks running
  length-wise.
• Serial recording - data are laid out as a
  sequence of bits along each track.
• Data are read and written in contiguous blocks,
  physical records.
• Block are separated by interrecord gaps.
• The tape is formatted to assist in locating
  physical records.
• Data are recorded serially along individual
  tracks.
• Blocks in sequence are stored on adjacent tracks.

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Magnetic Tape Characteristics

• A tape drive is a sequential-access device.
• Read all of the front records sequentially to get
  the desired one.
• Tape is in motion only during read or write
  operation.
• It is the first kind of secondary memory.
• Usually use for backup data from the system.
• Lowest-cost, slowest-speed memory

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Optical Types

• Compact Disk (CD)
• CD-ROM
• CD Recordable
• CD Rewritable
• Digital Versatile Disk (DVD)

18
Compact Disk CD-ROM

• CD digital audio system was introduced in 1983 as
  a nonerasable disk that can store more than 60
  mins of audio information.
• Audio CD and CD-ROM share a similar technology.
• CD-ROM player are more rugged and have error
  correction device.
• The disk is formed form a resin.
• Digitally recorded information is imprinted as a
  series of microscopic pits on the surface and
  then coated with a highly reflective surface
  (aluminum or goal) and then coated with acrylic.

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Compact Disk CD-ROM Layout Method

• CD and CD-ROM contain a single spiral track,
  begin near the center and spiraling out to the
  outer edge of the disk.
• Sectors near the outside are the same length as
  those near the inside.
• The information is packed in segments of the same
  size and these are scanned at the same rate by
  rotating disk at a variable speed, known as
  Constant Linear Velocity (CLV).
• The capacity of a track and the rotational delay
  increase for positions nearer the outer edge of
  disk.

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Compact Disk CD-ROM data Organization

• Data are organized as a sequence of block
• A typical block consists of
• Sync identifies the beginning of a block (12
  bytes).
• Header contain block address and mode byte (4
  bytes).
• Mode 0 - a blank data field
• Mode 1 - the use of an error-correcting code and
  2048 byte of data
• Mode 2 - 2336 bytes of data with no
  error-correcting code
• Data User data
• Auxiliary Additional user data in mode 2. In
  mode 1, 288-byte error-correcting code (ECC).

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Compact Disk CD-ROM Pros Cons

• Pros
• The optical disk can be mass replicated
  inexpensively.
• The optical disk is removable, allowing the disk
  itself to be used for archival storage.
• Cons
• It is read-only and cannot be updated.
• It has an access time much longer than that of a
  magnetic disk.

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Compact Disk CD Recordable (CD-R)

• A disk is prepared in the way that it can be
  subsequently written once with a laser beam with
  a more expensive disk controller than for CD-ROM
• The medium is similar to (not identical) that of
  CD or CD-ROM.
• Information is recorded by the the pitting of the
  surface, which changes reflectivity.
• The disk can be read on a CD-R or CD-ROM drive.
• It provides a permanent record of large volumes
  of user data.

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Compact Disk CD Rewritable (CD-RW)

• The disk can be repeatedly written and
  overwritten.
• Current materials can be used for
  500,000-1,000,000 erase cycles.
• It can be rewritten and used as a true secondary
  storage.
• It is higher reliability and longer life than
  magnetic disks.

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Digital Versatile Disk

• The industry has a found an acceptable
  replacement for the analog VHS video tape and
  replace the CD-ROM in PCs and servers.
• It can be randomly accessed like audio CDs.
• It can store data with vivid quality, currently 7
  times of CD-ROM.
• The greater capacity is due to
• Bits are packed more closely on a DVD (4.7 GB).
• It employs a dual-layer (8.5 GB), double sided
  disk (17 GB).
• DVDs come in recordable, rewritable as well as
  read-only version.