Storage

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Chapter 3

• Storage

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Overview

• This chapter covers
• Common characteristics of storage systems
• Magnetic disk storage systems
• Optical disc storage systems
• Other types of storage, such as flash memory,
  magnetic tape, remote storage, and smart cards

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Properties of Storage Systems Storage Devices
and Media

• Involve two physical parts a storage device and
  a storage medium.
• Can be internal (located inside the system unit),
  or external (located outside the system unit).
• Storage devices are identified by names and/or
  letters (e.g. C for first hard drive).

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Properties of Storage Systems Non-Volatility

• Storage media are non-volatile
• When power to the device is shut off, data stored
  on the medium remains.
• This is in contrast to most types of memory,
  which are volatile.

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Properties of Storage Systems Removable vs.
Fixed Media

• Fixed media typically faster and less
  expensive.
• Removable media unlimited capacity and can be
  easily transported and secured.

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Properties of Storage Systems Random vs.
Sequential Access

• Random access ( direct access ) data can be
  retrieved in any order, independent of its
  physical location (most types of storage media).
• Sequential access data can be retrieved only in
  the same sequence in which it is physically
  stored (magnetic tape).

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Properties of Storage Systems Logical vs.
Physical Representation

• Logical file representation refers to the users
  view of the way data is stored (filename,
  folders, etc.).
• Physical file representation is the actual
  physical way the data is stored on the storage
  media as viewed by the computer.

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Magnetic Disk Systems

• Magnetic disks are the most widely used storage
  medium in computers today.
• Data is stored by magnetizing particles on the
  storage medium.
• Two common types
• Floppy disks
• Hard disks

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Magnetic Disks and Drives, Contd.

• Disk is divided into tracks, sectors, and
  clusters.
• The disks file directory keeps track of the
  contents of the disk so files can be retrieved
  (by filename) at a later time.

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Magnetic Disks and Drives

• Floppy disk characteristics
• Typically 3½ inches in diameter.
• Typically hold 1.44 megabytes.
• Inserted into floppy disk drive to be read from
  or written to.

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Hard Disk Drives

• Hard drive characteristics
• Metal disk onto which data is stored
  magnetically.
• Disks are usually permanently sealed inside the
  hard driveallows faster speeds and storage of
  more data than removable systems.
• Can be internal or external.

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Hard Disk Drives, Contd.

• Hard drive characteristics, contd
• Organized into tracks, sections, clusters, and
  cylinders (the collection of tracks located in
  the same location on a set of hard disk
  surfaces).
• Read/write head doesnt touch the surface of the
  disk.
• Bumping PC when disk is being accessed or dust or
  other obstacles on a hard-disk system can cause a
  head crash and damage the surface of the disk.

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Hard Disk Drives, Contd.

• Disk access time.
• Factors seek time, rotational delay, data
  movement time
• Can use multiple partitions.
• Disk cachestrategy for speeding up system
  performance.
• Hard drive standards (EIDE, SCSI, Fibre Channel,
  USB) .

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Hard Disk Drives, Contd.

• Portable hard drive systems
• Offer large storage capacities and portability.
  
• Either entire drive or just hard disk cartridge
  is transported.

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Hard Disk Drives, Contd.

• Storage systems for large computer systems and
  networks
• Storage servers containing racks of hard drives.
• Network attached storage (NAS).
• Storage area networks (SANs).
• RAID (for increased performance and/or fault
  tolerance).

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Optical Disc Systems

• Laser beams write and read data packed at very
  tight storage densities, many times finer than
  that of a typical magnetic disk.
• Are typically 4½-inch discs, but can be a variety
  of shapes and sizes.
• Data is stored optically on a continuous spiral
  track.
• Can be CDs or DVDs.

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Read-Only Discs CD-ROM and DVD-ROM Discs

• CD-ROM discs cannot be written to or erased and
  typically hold 650 MB (e.g. music CDs).
• DVD-ROM discs are similar to CD-ROM discs, but
  have higher capacity of 4.7 GB to 17 GB (e.g.
  movie DVDs).
• Data is stored by burning pits into the disc
  surface that can be read using a laser beam.

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Recordable DiscsCD-R, DVD/-R

• Recordable discs can be written to, but not
  erased or rewritten.
• Burning a CD or DVD permanently records data onto
  the disc.
• CD-R discs are commonly used to store data and
  for custom music CDs.
• DVD-R/DVDR discs are commonly used for home
  movies and other high-capacity applications.

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Rewritable DiscsCD-RW, DVD/-RW

• Rewritable discs can be used in the same way as
  magnetic disksdata can be stored and erased as
  needed.
• Phase-change technology is used to record data
  onto the disc using reflectivity.
• Rewritable discs can be erased by reverting the
  reflectivity back to the original state.
• Blue laser discs are an emerging higher-capacity
  (23.3 GB) form of optical disc.

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Other Types of Storage Systems

• Flash memory media
• Chip-based storage.
• Solid state storage systemhas no moving parts,
  so are more shock-proof and portable than
  conventional storage systems.
• Commonly used with digital cameras, digital music
  players, handheld PCs, notebook computers, smart
  phones, etc.

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Other Types of Storage Systems, Contd

• Flash memory media, contd.
• Common forms of flash memory
• Flash memory sticks.
• Flash memory cardsCompactFlash, Secure Digital
  (SD), MiniSD, MultiMedia (MMC), SmartMedia, and
  xD Picture cards.
• Flash memory drives, such as USB flash drives.

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Other Types of Storages Systems, Contd

• Magnetic tape systemsplastic tape that is
  magnetized to represent data.
• Used primarily for backup and archival purposes
  (sequential access only).
• Read from and written to via a tape drive.
• Most tape media are in the form of cartridge
  tapes, though detachable-reel tapes exist as
  well.  

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Other Types of Storage Systems, Contd

• Remote storagestorage devices that are not
  directly connected to your PC.
• Network storageaccessible through a local
  network.
• Online storageaccessible via the Internet and
  used for backup, as well as to transfer files to
  others.

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Other Types of Storage Systems, Contd

• Smart card credit-card-sized piece of plastic
  that contains some type of computer circuitry.
• Holds less than a few megabytes.
• Commonly used to store prepaid amounts of digital
  cash or personal information.
• Smart card readers are attached to a PC or built
  into a cell phone, keyboard, or other device.

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Other Types of Storage Systems, Contd

• Holographic storage
• Uses multiple laser beams to store data in three
  dimensions.
• Stores data in page format.
• No moving parts and simultaneous access to all
  data on a page.

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Comparing Storage Alternatives

• Factors to consider speed, expense, portability,
  storage capacity, and compatibility.
• Most PC users require
• Hard drive
• CD or DVD drive
• Floppy drive
• Additional devices (flash memory card reader,
  etc.) as needed for the devices being used in
  conjunction with the PC (e.g. digital camera)

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Summary

• Properties of storage systems
• Magnetic disk systems
• Optical disc systems
• Other types of storage systems
• Comparing storage alternatives