Computer Storage

Computer Storage

Mikusch, Rita

Your computer’s storage capabilities largely determine the types of applications and programs you can use. It can therefore be useful to briefly discuss the main types of storage available for today’s computer systems.

Modern computer systems consist of four basic parts: the chip or central processing unit (CPU), the main memory (RAM), the storage (hard drives, CDs, diskettes), and the inputoutput devices (keyboards, mice, joysticks).

“Main memory” or RAM is not to be confused with storage-main memory is what a computer uses for the tasks it’s currently working on, while “storage” is used to store programs and data. Think of it this way: The computer first fetches the program it wants to run and the data it wants to use from the computer’s storage, then puts the program and data into its main memory to work on them.

Main memory is unsuitable for the longterm storage of programs and data because it’s volatile-when it loses power, all of the information stored in it disappears, making storage devices such as hard drives necessary. Main memory is also far more expensive than a hard drive, so you would never find a computer with enough RAM to hold all the programs and data required by its user(s).

Categorizing computer storage

There are a number of ways to categorize computer storage:

Primary vs. secondary storage: Confusingly, there are two completely different definitions for these terms:

(1) Primary storage stores information on hard drives and other devices that are immediately available to the computer, and secondary storage stores information on tapes and other devices that may be located far away from the computer (for example, back-up tapes and the computers from which the data on the tapes was copied are often stored in separate locations; or

(2) Primary storage refers to main memory, and secondary storage refers to the hard drives and other devices that store the programs and data.

Read-write vs. read-only: Read-write devices, such as hard drives and rewritable CDs, allow you to store and edit programs and data; read-only devices, such as read-only CDs (commercial music CDs, for instance), do not allow you to edit the information they contain.

Random access or sequential access:

Random-access devices allow you immediate access to any bit of data, while sequentialaccess devices require you to start from the beginning and read until you find the bit of data you’re seeking. An example of a random-access device is a hard drive (you can immediately access any item you’ve stored on it). An example of a sequential-access device is a data back-up tape (to access an item in the middle of the tape you have to start at the beginning).

Common storage devices

Diskettes: The first disk drives used 8″ floppy diskettes with a storage capacity of less than 100 kilobytes. In 1976,* the smaller 5 1/4″ floppy diskettes, which could hold up to 1.2MB, were introduced. The familiar 3 1/2″ floppy diskettes, which can hold up to 1.44MB, weren’t introduced until 1980. The 8″ and 5 14″ floppy diskettes are no longer in use, and the 3 1/2″ floppy diskettes are slowly being phased out-many new computer systems no longer come with compatible hardware.

Hard-disk drives: The hard drive is the primary storage device for desktop and laptop computers. A hard drive contains one or more magnetic disks that spin at high speed. The two significant measurements for a hard drive are the storage space and the speed in revolutions per minute (most drives today range from 4,500 to 10,000 rpm).

Hard drives are usually internal devices, with IDE and SATA being the most common internal connection types. Hard drives can also be external, but require that you use the right type of connection. Some of the options include: Fire Wire (also known as IEEE 1394), SCSI (pronounced “skuzzy”), and USB. SCSI is an older standard that is available for both internal and external drives, and is no longer in common use (except for some high-end servers). USB is a slower connection type and better suited for small flash drives (mentioned below) rather than regular hard drives.

Redundant Array of Inexpensive Disks

(RAID): In order to solve some of the speed, storage, and redundancy limitations of single hard drives, RAIDs can be used to combine multiple hard drives into a single unit. There are several different types of RAID set-ups. Two popular ones are RAID 1, which offers redundancy by recording all the information on two separate hard drives; and RAID 5, which offers greater storage and redundancy by distributing the data across three or more drives in such a way that no data will be lost if one of the drives should fail.

Tape drives: Tape drives are sequential storage devices, which makes them unsuited for immediate storage. However, their high capacity and cheapness (relative to a hard drive of the same size) makes them useful for long-term storage, such as data back-up.

CD/DVD drives: Both CD and DVD drives can be either read-only or readable and writable. They are commonly used for back-up and portable storage. I talked at length about CD storage and its reliability in the August 2005 issue of the magazine. With respect to recordable DVDs, there are several competing technologies. In order to find the right technology for you, consider discussing your requirements with your technical advisor.

Optical jukeboxes: CD and DVD drives are optical drives, and an optical jukebox is a collection of CD/DVD drives and disks that work together as a unit. Optical jukeboxes are very useful for organizations such as libraries that have a lot of stored information they want to make accessible to the public. One advantage of an optical jukebox is that the CDs and DVDs are not handled, which protects them from dirt and abuse.

Flash drives: The drives mentioned above are all mechanical devices; flash drives, on the other hand, are solid-state devices that have no moving parts. Per unit of storage space, flash drives are more expensive than other drives, but they’re very handy for small portable devices such as palm-sized computers and portable USB flash drives. USB flash drives are being used for “secure Exam” in this year’s UFE-software on the flash drive ensures the security of the laptop being used by the UFE writer and also regularly backs up the writer’s exam.

Other: A wide array of other storage devices has been created since the dawn of the computer age, including punch cards, paper tape, zip drives, magneto-optical drives, and jaz drives. Many of these devices have long since disappeared.

Backing up your data

No discussion on data storage is complete without mentioning data back-ups. Hard drives, the most common of all storage devices, are mechanical-the general rule of thumb is that one day your hard drive will fail and you will lose all your data. But if you’re lucky, you’ll have retired your hard drive before its “day of reckoning” comes. But since you can’t rely on luck, you’re better off putting together a data back-up strategy. There are a number of options available, such as RAIDs, tape back-ups, and writeable CDs. Which method you choose depends on your needs. Your best first option would be to consult your technical advisor.

* Zeytinci, Gamze. Evolution of Major Computer Storage Devices. From: teaching/papers/research/StorageDevices-Zeytinci.pdf.

By Rita Mikusch, Webmaster

Copyright Institute of Chartered Accountants of British Columbia Oct 2005

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