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Physical Storage Media
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Physical Storage Media

  • Physical Storage Media is used for storing (writing /recording/saving) and retrieving (reading/opening) data.

  • A storage medium (media is the plural) is the physical material on which data is stored.

  • A storage device is the computer hardware that records and retrieves data to and from a storage medium.

Classification

Classification of Physical Storage Media is based upon the following characteristics :

  • Speed of access
  • Cost per unit of data
  • Reliability
    • Data loss on power failure or system crash
    • Physical failure of the storage device
  • Can differentiate storage into :

  • Volatile Storage :

    • Loses contents when power is switched off
  • Non-volatile Storage :
    • Contents persist even when power is switched off.

Types of Storage Media

  • Cache
  • Main Memory
  • Flash Memory
  • Magnetic Disk
  • Optical Disk
  • Magnetic Tapes

Cache

  • Most costly and fastest form of storage.
  • Fast access, volatile.
  • Very small in size.
  • High speed memory

Main Memory

  • Fast access (10s to 100s of nanoseconds)
  • Too small or too expensive to store the entire database.
  • Volatile memory - Contents of main memory are usually lost in a power failure or `crash'.
  • Computer can Manipulate only data that is in main memory.
  • Therefore, every program you execute and every file you access must be copied from a storage device into main memory
  • Ex : RAM

Flash Memory

  • Non-volatile - Data survives power failure
  • Reads are roughly as fast as main memory.
  • But writes are slow (few microseconds), erase is slower
  • Widely used in embedded devices such as digital cameras, USB flash drives, video games, etc..

2 Types :

  • NAND Flash
  • NOR Flash

Magnetic Disk Storage

  • Data is stored on spinning disk, and read/written magnetically
  • Primary medium for long-term storage.
  • Typically the entire database can be stored on disk.
  • Data must be moved from disk to main memory for access, and written back for storage.
  • Disk storage is called direct access storage as it is possible to read data on the disk in any order.
  • Disk storage usually survives power failures and system crashes (non-volatile).
  • Ex : floppy disks, hard disks, etc.

Optical Storage

  • Non-volatile, data is read optically from a spinning disk using a laser
  • CD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular forms
  • Blu-ray discs : 27 GB to 54 GB
  • Write-one, read-many (WORM) optical discs used for archival storage (CD-R, DVD-R, DVD+R)
  • Multiple write versions also available (CD-RW, DVD-RW, DVD+RW, and DVD-RAM)
  • Reads and writes are slower than with magnetic disk

Tape Storage

  • Non-volatile, used primarily for backup (to recover from disk failure), and for archival data

  • Sequential-access - much slower than disk

  • Very high capacity (40 to 300 GB tapes available)

  • Tape can be removed from drive ⇒ storage costs much cheaper than disk, but drives are expensive

  • Tape jukeboxes available for storing massive amounts of data

    • hundreds of terabytes (1 terabyte = 10⁹ bytes) to even multiple petabytes (1 petabyte = 10¹2 bytes)
  • Primary storage : fastest media but volatile (cache, main memory).

  • Secondary storage : next level in hierarchy, non volatile, moderately fast access time

    • E.G. Flash memory, magnetic disks
  • Tertiary storage : lowest level in hierarchy, non volatile, slow access time

    • Also called off-line storage

Magnetic Disks

  • Magnetic disks provide the bulk of secondary storage for modern computer systems.

  • The storage requirements of large applications have been growing very faster than the growth rate of disk capacities.

Platter

  • Each disk platter has a flat, circular shape.
  • Its two surfaces are covered with a magnetic material, and information is recorded on the surfaces.

    Read-write head :

  • Positioned very close to the platter surface (almost touching it)

  • Reads or writes magnetically encoded information.

Tracks :

  • Surface of platter divided into circular tracks
  • Over 50K-100K tracks per platter on typical hard disks

Sectors :

  • Each track is divided into sectors.
  • A sector is the smallest unit of data that can be read or written.
  • Sector size - typically 512 bytes
  • Typical sectors per track : 500 to 1000 (on inner tracks) to 1000 to 2000 (on outer tracks)

To read/write a sector :

  • Disk arm swings to position head on right track

  • Platter spins continually; data is read/written as sector passes under head.

Head-disk assemblies :

  • Multiple disk platters on a single spindle (1 to 5 usually)
  • One head per platter, mounted on a common arm.
  • Cylinder i consists of ith track of all the platters

What is a cylinder?

  • Vertical section of track through all platters

  • Single movement of read/write head arms accesses all platters in cylinder

Disk Controller :

  • Interfaces between the computer system and the actual hardware of the disk drive.

  • It accepts commands to r/w a sector, and initiate actions.

  • Disk controllers also attach checksums to each sector to check read error.

  • Remapping of bad sectors : If a controller detects that a sector is damaged when the disk is initially formatted, or when an attempt is made to write the sector, it can logically map the sector to a different physical location.

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