Secondary Storage

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Secondary Storage

• Lecture 7

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Competencies

• Why do we need secondary storage?
• Describe floppy disk and compare to Zip Disks,
  SuperDisks, HiFD disks
• Describe internal hard disks, hard-disk
  cartridges, and hard-disk packs.
• Describe ways to improve hard-disk operations
  disk caching, redundant arrays of inexpensive
  disk, and data compression.

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Competencies

• Describe Pendrives
• Compare the CD and DVD optical disk formats.
• Describe the different types of optical disk.
• Describe magnetic tape streamers and magnetic
  tape reels.

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Why do we need Secondary Storage?

• Memory is limited in size and it is temporary
  (volatile).
• Secondary storage holds information external from
  the CPU.
• Secondary storage allows you to store programs,
  data, and information permanently.

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Two main techniques

• Magnetic
• Optical

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Types of Secondary Storage

• Floppy Disks
• Hard Disk
• Pendrives
• Optical Disks
• Magnetic Tapes

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Disk Structure

• A phonograph record is a long spiral, beginning
  at the outer edge of the record and ending near
  the label in the center.
• Disk structure is like a dart-board (concentric
  circles on a bulls-eye target).
• Data is stored in chunks, called sectors.

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Disk Structure

• It may take many sectors to store one piece of
  data (what you are saving will probably be bigger
  than will fit in one sector).
• A group of sectors is called a block.
• Blocks/sectors are used because data changes.

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Floppy Disks

• Portable, removable storage media.
• Also known as diskettes, disks, flexible disks,
  and floppies.
• They are flat circular pieces of plastic that
  rotate within a jacket.
• Can store 1.44 megabytes of data equivalent of
  400 typewritten pages.

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Floppy Drive
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Diskette anatomy
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Write-protect feature
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Other types of Floppy Disks

• Thicker and require special disk drives
• Zip Disks 250 MB
• Cannot read 1.44 MB floppy disks
• SuperDisks 120 MB
• Can read usual floppy disks
• HiFD disks 200 MB
• Can read usual floppy disks

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Hard Disks

• A hard disk is another type of permanent storage.
• Hard disks use thick, ridged metallic platters as
  their storage media.
• They are able to store and retrieve information
  much faster than floppy disks.
• They have a greater storage capacity than floppy
  disks.

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Microcomputer internal hard-disk drive

• (Top left) A hard-disk drive that has been
  removed from the system cabinet.
• (Middle right) Anatomy of a hard-disk drive.
• (Bottom left) The hard- disk drive is sealed
  inside the system cabinet and is not accessible.
  The drive gate is for inserting a diskette.

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Hard Disk Crash

• The read-write head of a hard disk drive rides on
  a cushion of air about 0.000001 inch thick.
• The cushion of air is so thin that certain
  circumstances make the disk vulnerable to data
  loss.
• Smoke particles, fingerprints, dust, or human
  hair can come between the read-write head and the
  magnetic disk surface.
• This could cause some or all of the data on the
  disk to be destroyed and is referred to as a disk
  head crash.

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Materials that can cause a head crash
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Hard-Disk Cartridges

• Easy removal
• Complements internal HDD
• Iomega and SyQuest

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Hard-Disk Packs

• Removable, massive storage capacity
• Common in mainframes
• Resembles stack of vinyl records

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Defragmentation

• When the disk fills up, the operating system
  stores data in whatever free space is available.
  Thus, files become fragmented.
• The read/write head must go through extra
  movements to find data, thus slowing access to
  data.
• Defragmentation means that data on the hard disk
  is reorganized so that data in each file is
  stored in contiguous clusters.

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Computer access time

• The time between the computers request for data
  from secondary storage and the completion of the
  data transfer.

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Performance Enhancements

• Disk caching
• Redundant arrays of inexpensive disks (RAID)
• File compression and decompression

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Disk caching

• It improves performance by anticipating data
  needs.
• It requires a combination of hardware and
  software.
• Frequently used data is read from the hard disk
  into cache memory during idle processing.

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Disk caching

• Data is then accessed directly from cache memory
  to RAM memory when needed.
• The transfer rate from cache memory is faster
  than from the hard disk.
• System performance can increase as much as 30
  percent using disk caching.

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RAID (Redundant Arrays of Inexpensive Disks)

• RAIDs are inexpensive hard-disk drives grouped
  together using networks and special software.
• They improve hard disk performance by expanding
  external disk storage.
• The grouped disks are treated as a single
  large-capacity hard disk.
• Sends data to the computer along several parallel
  paths simultaneously.
• RAIDs can outperform single disks of comparable
  capacities.

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RAID

• Sends data to the computer along several parallel
  paths simultaneously.
• RAIDs can outperform single disks of comparable
  capacities.

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Pen Drives

• Pen Drive is a USB FLASH MEMORY DRIVE
• It can support up to 8GB disk space, which is
  5600 times more than a 1.44MB floppy disk!
• It is a plug and play device.
• You can read, write, copy, delete and move data
  from your hard disk drive to the Pen Drive or
  from the Pen Drive to your hard disk drive.
• It does not require any battery (for USB port),
  it does not need any software or cables.

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Pen Drives

• Compact
• Plus MP3 player 4 in 1 - integrated MP3- / WMA
  player FM-radio USB memory stick
• 18 hours of music and fun (at 128 kpbs, on 1xAAA
  battery), powersaving mode
• LCD-Display with 96 x 32 mm
• Equalizer
• Control Menu

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Pen Drive Camera

• 2 in 1 - Pen Drive USB Stick Handy Cam
• Real 16001200 pixel without interpolation - 2
  Megapixel
• 2 AAA batteries

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File Compression

• It can increase storage capacity by reducing the
  amount of space required to store data and
  programs.
• File compression can be used with hard or floppy
  disks.
• It helps to speed the transmission of files from
  one computer to another.
• It is the common file transfer activity over the
  Internet.

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File Compression

• File compression replaces repeated bit patterns
  with a token symbol to reduce space.
• Files can be shrunk to a quarter of original size
  using file compression.
• Files can be decompressed back to original file
  with decompression software.

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Optical Disks

• Compact, permanent storage (CD and DVD)
• Laser beams reflect off pits, reading data up to
  4.7 gigabytes
• Equivalent of over 1 million typewritten pages
• Three types of CDs
• CD-ROM, CD-R, CD-RW

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Optical disks

• (Top) Writing data a high-powered laser beam
  records data by burning tiny pits in an encoded
  pattern onto the surface of a disk.
• (Bottom) Reading data a low-powered laser beam
  is used to read data because it reflects off
  smooth areas (lands), but does not reflect off
  pitted areas. These lands and pits are converted
  into 1 bits and 0 bits.

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CD-ROM

• Compact disc read-only memory
• Not written to
• 650-700 MB capacity
• 269,000 pages of text or more than 7,500 photos
• Speed how fast data can be transferred from the
  disk
• 24X 3.6 MB per second
• 32X 4.8 MB per second

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CD-R CD-RW

• CD-Recordable or WORM (write once, read many)
• CD-RW or rewritable optical discs

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CD-R for Music

• Download from Internet
• MP3
• Arrange and play music
• WinAmp
• Create custom CD-R or CD-RW
• Easy CD Creator by Adaptec

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DVD

• Digital Versatile Disk or Digital Video Disk
• Up to 4.7 gigabytes a capacity, seven times CD
• DVD-ROM (2 hours of very high quality video and
  sound major impact on the video market)
• DVD-R (higher cost than CD-R)
• DVD-RAM or DVD-RW (reusable)

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DVD versus CD-ROM

• Why DVD can hold more data

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Magnetic tapes

• stores data represented by magnetized particles
  in linear tracks
• magnetized clusters or domains are aligned to
  represent binary codes

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Magnetic tape
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Magnetic Tape

• Fast, direct access
• Sequential access
• Storage and backup
• Magnetic tape streamers
• Magnetic tape reels

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Magnetic Tape Streamers

• Backup tape cartridge units
• Capacities range from 120 MB to 5 GB
• Digital audio tape (DAT)

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Magnetic Tape Reels

• Used with minicomputer and mainframe computers
• Often 1/2 inch wide and 1/2 mile long
• Stores 1600 to 6400 characters per inch
• Tapes are run on magnetic tape drives or magnetic
  tape units

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Magnetic Tape Reels
Take-up reel
Supply reel
Tape
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Memory Hierarchy
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Memory Hierarchy