Secondary storage

• How does a Solid State work?


  Referred to as flash storage or flash memory

  • Storing a bit - Made of flash memory, stores the value of each bit in a semi-conductor chip
  • Accessing data - Can be accessed randomly. Any piece of data can be accessed in a constant amount of time - achievable because there are no moving parts
  • Reliable - SSDs are robust, no moving parts - have a limited number of read/write cycles.
• Properties of SSMs?


  • High speed - Solid State Media tends to have fast read/write times, some have slower e.g. USB sticks
  • High cost - Solid State media has a higher cost-per-GB than magnetic media - SSDs do not last as long as HDD so the cost seems restrictive
  • Good capacity
  • Good reliability - Robust, rarely fails (until read/write limit is hit) - not easily damaged by shocks
• How does Optical Storage work?


  Uses a laser light to retrieve data from the surface of optical media. Used to deliver multimedia content such as video.

  • Storing a bit - Each 1 or 0 is represented as a microscopic hole (called a pit) or a flat surface (called a land) on the surface of the disk
  • Accessing data

  1. If the laser hits a land, it will be reflected in the sensor
  2. If it hits a pit, then it will be reflected somewhere else
• Properties of optical media


  • Low cost - Very cheap per-gigabyte
  • Great reliability - Waterproof, shock proof
  • Portable
  • Low capacity - Disks are small
  • Low speed - Because the parts in the optical media drive have to move to find the right bit of data on the disk, access times can be quite slow
• Optical storage - Compact Disk (CD)


  • The original optical media standard
  • Can store 700mb of data
  • Cheap to produce
• Optical storage - Digital Versatile Disk (DVD)


  • Used to store games or standard-definition movies
  • Can store 4.7 GB of data
• Optical storage - Blu-Ray Disks (BD)


  • Often used to store high-definition games or movies
  • Store 25 GB of data
• Optical disk - Read-Only Media (ROM)


  • A CD-ROM, DVD-ROM or BD-ROM is a disk which can be read only
  • Manufacturer writes the initial data, and this cannot be changed
  • This is often used in content distribution so that people cannot accidentally overwrite the content
• Optical disk - Write-One (R)


  • CD-R, DVD-R or BD-R is a disk which can be written once by the user at home
  • Once the user has written data to that disk it cannot be overwritten
• Optical disk - Re-writable (RW)


  • CD-RW, DVD-RW or BD-RW is a disk which can be rewritten whenever they want
  • These are often for backups
• How does magnetic storage work


  • Storing a bit - Value of each bit of data is represented as positively or negatively charged magnetic particle. Particle are part of a magnetic disk called the platter
  • Accessing data - Data could be stored anywhere on the disk. To read/write data it needs to first locate it. A magnetic read/write head is used to access the data while the platter spins
  • Reliability - HDD has moving parts. Subject to wear-and-tear. Not robust
• Properties of Magnetic Media


  • Low cost - Good for storing large amount of data
  • Low speed - Needs a high number of moving parts.
  • Good reliability - Lasts a long time. Not robust
  • High capacity - Popular choice for archives and home computers
• Hard Disk Drive (HDD)


  • Often used as the main form of secondary storage in a computer
  • HDDs hold the operating system, applications and user data
• Hybrid drives


  • Some modern devices have both magnetic and solid state drives
  • Combination of these is know as a hybrid drive
• Secondary storage table

  A) Flash Drive

  B) External hard drive

  C) CD/DVD/Blu-ray disc

  D) Magnetic tape

  
• Optical storage


  Optical storage media uses technology such as lasers. Laser beams are projected onto a CD/DVD or Blu-ray disc and if light is reflected back, then data is read as a 1. If light is not reflected back, data is read as a 0. Lasers are used to read and write information on a disk.
• Magnetic storage


  This technology is used in hard disks and tapes. Data is stored on a magnetic medium, which can be a disc or a tape, by writing data using a write-head. Data can then be read by the read-head.
• Solid State


  Solid state technology is used in storage media such as flash memory sticks. The technology is called solid state as it doesn't have any moving parts, such as a read-head in magnetic storage.