Topic 6 - Resource Management [HL]

Secondary Storage Devices

This section compares the most common types of secondary storage devices.and their use cases.

Magnetic Media

Fixed hard drive (HDD)

Data are stored on the surface of metal discs which have a magnetisable coating. Dots on the disc can store different magnetic field values (1 or 0). The dots are arranged in circles and sectors along the disc and can be read by the head, while the disc spins at high speeds.

Application: Main backing storage for almost all computers, as they offer random access and relatively high access speeds.

Typical Capacity: 1-4TB

Portable hard drive

Same function as fixed HDD, although they are smaller and come with electronics that allow the drive to be accessed via USB or a similar connection.

Application: The allow the easy transportation of very large amounts of data from one computer to another.

Typical Capacity: 500GB-2TB

Magnetic tape drive

Data are also stored on a magnetised film, however these are arranged along the length of a long plastic strip. Data are accessed serially, accessing individual files is therefore slow.

Application: Tapes are used mainly for data back-ups where large amounts of data need to be stored, but quick access to individual files is not required. Tapes are also used in some batch-processing applications (e.g. to hold the list of data that will be processed).

Typical capacity: Potentially up to 185 TB

Optical media

All of these work in a similar way, in which data are stored as a pattern of dots that can be read by light, which is usually a laser beam. The data are read by bouncing the laser beam off the surface of the medium. The different dot patterns can be read as the reflect the laser beam differently. All of these allow random access but differ in their capacity and if they can be written.

CD-ROM/R/RW

  • ROM: read-only
  • R: recordable
  • RW: rewritable

Application: content distribution, e.g. music, software or e-books. Cheap writable storage.

Typical capacity: ~800 MB

DVD-ROM/R/RW/RAM

  • ROM: read-only
  • R: recordable
  • RW: rewritable
  • RAM: high-quality rewritable with high reliability

Application: distribution of high quality videos (movies, documentaries), re-usable storage medium, data backup and archiving.

Typical capacity: ~4.7 GB / 8.5 GB (dual-layer)

Blu-ray disc

They work in the same way as DVD-ROMs, but due to a shorter laser wavelength they have a higher capacity.

Application: Distribution of HD videos and big software, such as console games.

Typical capacity: 25-50 GB

HD-DVD

Meant as a competition to the Blu-ray disc, but less established.

Application: Same as Blu-ray, but no longer in use.

Typical capacity: 15/30 GB

Solid state memory

Flash memory is a non-volatile storage medium which can be electrically be erased and reprogrammed and is based on EEPROM (electronically-erasable programmable read-only memory). The most common type is NAND, where data can be read or written in blocks.

USB flash drive

This data storage device uses flash memory which can be connected to a computer via USB.

Application: Used to transport small amounts of data quickly between different computers, specially because they have a universal compatibility and are easy to transport(small).

Typical capacity: 2-128GB (although up to 1TB exists)

Memory card

Memory cards use the same flash memory technology, but in form of a card.

Application: Used mostly in portable electronic devices as digital cameras, mobile phones, laptops, tablets, etc.

Typical capacity: 1-128GB

Solid State Drive (SSD)

Is based on the same flash memory technology, but its interface is the same as of a hard drive (most commonly SATA).

Application: Used mainly as a main storage for desktops and laptops, sometimes in combination with HDDs where larger capacities are needed. SSDs are best used for data that require fast access times and read/write rates as for booting the operating system or starting important programs.

Typical capacity: 128GB - 1TB