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Candidates should be able to:
- explain the need for secondary storage
- describe common storage technologies such as optical, magnetic and solid state
- select suitable storage devices and storage media for a given application and justify their choice using characteristics such as capacity, speed, portability, durability and reliability.
Why is secondary storage needed?
Although RAM can store the data and program files used when a computer system is actually running, it is volatile, meaning it loses its contents without a power source.
A secondary storage device is needed in a computer system to store data and program files when the power supply is turned off. Secondary storage must therefore be non-volatile, meaning it retains its contents without the need for a power supply. Secondary storage may also need to have a larger storage capacity than RAM if it needs to store operating system (OS) files, other program files and data files.
When a computer is running, the programs and data files in use are copied from the secondary storage into RAM (loading) so the CPU can access them at an acceptable speed. Any changes to the files are copied back to secondary storage from RAM before the computer is closed down (saving).
Secondary Storage compared to Memory
- It is always slower to access data from secondary storage than from memory.
- Data stored in secondary storage is permanent, it is NOT lost when the computer is turned off.
- Secondary storage typically has a far higher storage capacity than RAM.
What are the common storage technologies?
Secondary storage can be divided into three main technologies:
- These have a tape or disk surface which can be magnetised with a pattern of north and south areas to represent the binary data. As the changing magnetic surface moves past a read head it reads the alternating pattern of tiny current changes as binary digits.
- Typical examples include hard drives, floppy disks and magnetic tape.
- These use a circular disk surface where a spiral pattern of reflective and non-reflective areas represents the binary data as the disk surface moves past it.. A low-power laser beam tracks this spiral pattern and reflects laser light onto a light detector. The pattern of bright and non-bright reflections form a pattern which is read as binary digits.
- Typical examples include CD-ROM, CD-R and CD-RW disks, along with the higher capacity DVD and Blu-ray equivalents.
Solid state devices
- These store the binary data on semi-conductor material where a pattern of on/off semi-conductor switches represents the binary data.
- A typical example is a solid state drive.
What applications are different storage devices and storage media suited to?
Data is actually stored on the storage media (some is removable and some is fixed into the computer). The storage device is the device that actually reads data from, or writes data to, the storage media.
Magnetic media – Hard Drives
In these devices the binary data is stored on the magnetised surface in a circular pattern on the surface of flat, circular plates called platters. These rotate at very high speed and a read/write head moves from side to side to load and save data.
Files are stored on the disk in clusters, arranged along tracks and spanning one or more sectors. The tracks and sectors are created when the hard disk is first formatted and this must take place before the disk can be used.
For a drive to read data from a disk, the read/write head must move sideways to align with the correct track (the time to do this is called the seek time) and then wait until the correct sector rotates round until underneath it.
- Applications: The hard drive is usually the main secondary storage media for a typical computer or server. It is used to store:
- The computer operating system (OS) (e.g. Microsoft® Windows, Linux etc. )
- Applications software files (e.g. word-processor, database, spreadsheet, etc.)
- Data Files such as documents, music, video etc.
- A normal home/school microcomputer would have a hard drive with a capacity of over 300 gigabytes.
- Very fast access to data as it can be read directly from any part of the hard disk (random access) and the disk is constantly spinning while the computer is in use.
- Relatively cheap in terms of capacity per unit price.
- Very reliable, lasting for many years in typical use.
- Very large capacity, significantly larger than any other form of secondary storage.
- Usually not portable (although removable versions do exist, mainly as backup devices).
- Vulnerable to damage if moved too violently, particularly while reading/writing data which can damage the read/write head.
Magnetic media – Floppy disks
Rarely used now but 3.5 inch floppy disk drives can still be found on some early computers. The floppy discs for a PC needed to be formatted before they were used but were usually pre-formatted to hold 1.44 MB of data (enough to store about 350 pages of A4 text).
- Applications: Floppy discs are removable media so are useful for transferring small amounts of data between computers and for keeping a back-up of small files. However, no longer used in favour of solid state storage.
- They are very cheap to buy but floppy disc drives are becoming less common.
- Data can be read directly from any part of the disk (random access).
- Portable but tiny small storage capacity compared to modern solid state alternatives such as USB memory sticks.
- Not reliable as they are easily physically damaged if unprotected and magnetic fields can damage the data.
- Very slow to access data because floppy discs rotate far more slowly than hard discs and only start spinning when requested.
Magnetic media – Magnetic Tape
Just like the tape in a tape-recorder, the data is written to or read from the tape as it passes the magnetic heads. This means that it is necessary to start at the beginning of the tape and search for the data as the tape goes past the heads (serial access) which is considerably slower than random access (being able to jump to a selected section of data).
- Applications: Magnetic tapes are typically used to make a copy of hard discs as a back-up. This is often done automatically overnight and the tapes kept in a safe place away from the server.
- A typical data tape and drive:
- Magnetic tape is relatively cheap and tape cassettes can store very large quantities of data (typically 26 GB).
- Portable and very reliable.
- To access data, the tape has to be read from the beginning until the data is found which is very slow (serial access) and means it is not possible to go directly to an item of data on the tape as with a magnetic disk or optical media.
Optical Media – CD-ROM / CD-R / CD-RW
Like a floppy disc, all CD’s (Compact Disks) are removable media and, unlike hard drives, only start spinning when requested. This means that, although they are much faster to access than a floppy, they are still a lot slower than a hard disc.
As the disk spins, a low-power laser beam is moved over a spiral pattern of reflective or non-reflective areas on the disk surface and the pattern of reflected light allows the binary data to be read. Data can be read directly from any part of the CD (random access).
- CD-ROM: (Compact Disc – Read Only Memory). This media is read only, the data cannot be altered or added to. Binary data is stored as a spiral pattern of reflective and non-reflective bumps (moulded onto the the surface when the disk is manufactured) and is read by a low power laser reflecting or not reflecting light off the surface.
- Applications: Most software programs are now sold on CD-ROM.
- CD-ROM’s hold relatively large quantities of data (650 MB) and CD drives are universally readable in all modern PCs.
- The data cannot be accidentally deleted or edited as they are read-only media.
- Cheap, portable and relatively fast.
- You cannot edit or add to the data on a CD-ROM.
- CD-R: (Compact Disc – Recordable). This media is recordable, data can be written to the disk but not edited. The data capacity is the same as for a CD-ROM. Binary data is stored as a spiral pattern of opaque and non-opaque areas on the surface. To record data, a high power laser beam alters the colour of a dye layer to create the opaque and non-opaque areas.
- Applications: Backing up computer data, creating music CD’s at home.
- As for CD-ROM, with the additional advantage of being able to record data.
- You cannot edit data already recorded on a CD-R, only delete it or add further data to it.
- The dye used to record the binary data can degrade over time, especially in bright sunlight.
- CD-RW: (Compact Disc – Read Write). This media is recordable and the data can be edited making them reusable. The data capacity is the same as for a CD-ROM. Binary data is stored as a spiral pattern of reflective and non-reflective areas on the surface, different heat settings from a high power laser cause the surface to switch between these two states.
- Applications: Backing up computer data, creating music CD’s at home, transferring files.
- CD-RW offer similar advantages to CD-R disks with the added advantage that they are reusable.
- They are also more stable than CD-R in sunlight.
- Relative easy to scratch the surface causing read/write errors.
Optical Media – DVD / Blu-ray
- A DVD (Digitally Versatile Disk) offers a higher capacity (typically 4.7GB) than a CD (Compact Disks).
- A Blu-ray disk offers an even higher capacity (typically 25GB)
- DVD’s and Blu-ray disks are both removable media and work in the same way as CD’s. They are both physically the same size as CD’s so the higher capacities are achieved because the reflective and non-reflective areas are closer together and the spiral pattern that these areas are arranged in is tighter. As with CD’s, data can be read directly from any part of the disk (random access). Recordable and read/write versions are available.
- Applications: Storing movies that are viewed using a DVD player or computer. Some large multimedia-heavy software games are now sold on DVD. Backing up larger amounts of data.
- As for CD’s with the additional advantages that DVD’s have a relatively high capacity and DVD drives are now common in modern PCs.
- Portable and relatively fast.
- Disadvantages: As for CD’s. DVD recorders are more expensive than their CD equivalents.
- Blu-ray disk:
- Applications: Storing high definition movies that are viewed using a Blu-ray player or computer.
- As for CD’s with the additional advantages that Blu-ray disks have an even higher capacity.
- Portable and relatively fast.
- Disadvantages: As for CD’s. Also, many computers do not have Blu-ray players as standard and recordable versions are even less common.
Solid state devices – Solid state drives
- Solid state devices are a form of flash memory, it is therefore non-volatile like ROM so the contents are not lost when there is no power supply, but able to be written to as in RAM.
- Applications: Solid State Devices can replace the hard drive as the main secondary storage device in a computer, where really large storage capacities are not required. They form the main storage for PDA, smartphones and some high-end laptop models.
- Solid State Devices offer faster read/write times than either magnetic or optical storage devices.
- They are also silent and resistant to physical shock which makes them ideal as portable storage devices.
- They offer relatively high storage capacities and data can be read directly from any part of the device (random access).
- They also can be designed to use the same interface as hard drives, thus easily replacing them in many applications.
- The storage capacity cannot yet match that of modern hard drives.
- Relatively expensive compared to magnetic media in terms of data capacity per unit price so they tend to be smaller capacity than magnetic storage devices.