Storage Devices

Storage Devices

Storage Device – storage medium and mechanism for storing and retrieving data on that medium.

Storage Medium – physical material that holds data presentation.

Benefits of Storage Devices

A.      Space

Ø  Diskette contains equivalent of 5000 printed pages.

Ø  Optical Disk can hold 500 books.

B.      Reliability

Ø  Relatively safe.

C.      Convenience

Ø  Easy and quick location of data.

D.      Economy

Ø  Saving in storage cost.

Types of Storage Devices

A.      Volatile – data presentation are erased when electric power is turned off.

B.      Non-volatile – data presentation is retained even without power.

Secondary Storages

A.      Magnetic Disk Storage

a.       Diskettes – three and a half inches diskette holds 1.44MB of data.

b.      Hard Disk

Ø  How data is organized:

a.       Track – circular portion of the disk surface that passes under the read/write head. Floppy diskette has 80 tracks on each surface. Hard disk may have 1,000+ tracks on each platter.

b.      Sector – each track is divided into sectors that hold a fixed number of bytes. 512 bytes per sector.

c.       Cluster – a fixed number of adjacent sectors that are treated as a unit of storage. Typically two to eight sectors, depending on the operating system.

d.      Cylinder – the track on each surface that is beneath read/write head at a given position of the read/write head.

Ø  Data Transfer (measure of performance)

a.       Average access time

b.      Data transfer rate

A.      Optical Disk Storage

a.       Compact Disk

§  CD-ROM – drive can only read data from CDs. Stores up to 700MB per disk.

§  CD-R – recordable. Write disk only once.

§  CD-RW – rewritable. Can erase and record over data multiple times.

b.      Digital Versatile Disk

§  Capacity up to 17GB.

§  Allows full length movie.

§  Sounds better than on audio CD.

c.       Magnetic tape storage

§  Similar to tapes used in music cassettes.