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.