Secondary Storage


The Two Main Tiers

  • Primary Storage refers to the memory areas that the CPU can acces very quickly, lie CPU registers, cache, ROM annd RAM. Prmary storage has the fastest read/write speed and is mostly volatile.
  • Secondary Storage is non-volatile - it`s where all data (operating systems, applications and user files) is stored when not in use. It includes magnetic hard disk drives, solid state drives, CDss and SD cards. Read/write speeds are much slower compared to primary storage.
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Magnetic Hard Disks

  • Hard Disk Drives (HDDs) are the traditional storage in PCs and laptops.
  • A hard disk drive is made up of a stack of magnetised metal disks spinning at a rate of between 5400 and 15000 rpm (revolutions per minute).
  • Portable HDDs are popular for backing up and transporting large amounts of data.
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Magnetic Hard Disks continued

  • Data is stored magnetically in small areas called sectors within circular tracks. Read/write heads on a moving arm are used toaccess sectors on the disks.
  • Despite their moving parts, HDDs are generally very long lasting and reliable, although they could be damaged by large impacts like being dropped.
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Advantages of HDDs

  • HDDs are cheaper.
  • Both are high capacity, but HDDs are higher.
  • HDDs have a longer read/write life than SDDs - SDDs can only be written on a certain number of times before they begin to deteriorate.
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Solid State Drives

  • Solid State Drives (SSDs) are storage devices with no moving parts. Most of them use a type of flash memory. SSDs are used for the same purpose as HDDs - for internal storage.
  • Like HDDs, Portable SSDs can be used to back up and transport data.
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Solid State Drives continued

  • SSDs have significantly faster read/write times than HDDs Using a SSD rather than traditional HDD can give much quicker times for booting up and opening programs and files.
  • Hybrid drives exsit which use solid state storage for the OS and programs, and a hard disk for data.
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Advantages of SSDs

  • SSDs are faster.
  • SSDs don`t need defragmenting.
  • SSDs are more shock proof than HDDs.
  • HDDs make some noise, SSDs are silent.
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Other types of flash storage

  • USB pen drives an memory cards are alsoo flash-based, solid-state storage.
  • They`re much slower than SSDs and have a  much shorter read/write life.
  • They`re used to expand the storage capacity of small devices like cameras, smartphones and tablets (which are too small for SSDs or HDDs). Their capacity is very high relative to their tiny size.
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Optical Discs

  • Optical discs are things like CDs, DVDs and Blu-Ray discs.
  • CDs can hold around 700 MB of data, DVDs can hold around 4.7 GB and Blu-Rays can hold around 25 GB.
  • Optical discs come in three forms:
    • Read-only (e.g. CD-ROM/ DVD-ROM/ BD-ROM)
    • Write-once (e.g. CD-R/ DVD-R/ BD-R)
    • Rewritable (e.g.  CD-RW/ DVD-RW/ BD-RW)
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Optical Discs continued

  • They do have some advantages, they`re very cheap (per GB), portable, won`t damage by water or shocks (although they can easily scratched).
  • Nowadays, their use is declining:
    • Modern devices like phones and tablets don`t have optical drives.
    • They can`t compete with flash storage devices due to their low capacity, very slow read/write speeds and poor reliability of RW drives.
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Magnetic Tapes

  • Magnetic tape has much greater storage than HDDs. It also has an extremely low cost per GB.
  • Magnetic tapes are often used by large organisations in archive librares to store huge amounts of data.
  • Tape is read/wrotten sequentially, meaning it is read/written from beginning to the end, or until it is stopped by the computer. This means tape is very slow when finding specific data stored on it, but has a fast read/write speed once it is in the correct place to begin reading/writing.
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