Secondary Storage

Examples of storage devices include CD, DVD, Bu-ray, USB/flash drive, SD card, portable solid-state drive, internal solid-state drive, magnetic hard drive, mass storage tape, external hard drive, cloud storage.

Characteristics of a Hard Disc Drive (HDD)

• A common speed for hard drives is 7200 RPM, but it can vary.

• Hard drives have a fast transfer rate and a fairly fast access time, they provide a good compromise between storage capacity, performance and cost.

• Their speed does not come close to the speed of memory, the CPU or SSD.

• Hard drives are a magnetic medium and store data on a hard drive platter.

• Data is read and saved using an arm that has a special read/write head at the end.

• As the disk spins, the arm travels across the disk.

• Each sector of the platter can store data and the movement of both the disk and the read/write head means that every sector on the hard drive can be reached.

• The faster the platter spins, the faster data can be read from the disk. (This speed is measured in revolutions per minute, or RPM.)

• A common speed for hard drives is 7200 RPM, but it can vary.

• Can be used as a hybrid with SSD.

• Fragmentation can slow access speeds.  Fragmentation means that the related data is split and stored on different parts of the disk.  

Effect = When data is fragmented, it takes longer for the disk heads to move between parts of the file, which slows the process of loading it.

DEFRAGMENTATION - Defragmentation  is the process where files are physically rearranged on disk so that they are no longer fragmented and the parts of each file are stored together. This improves the speed of accessing data from disk. 

* Their speed does not come close to the speed of memory, the CPU or SSD because it has moving parts e.g. read/write head.  HDD are less durable than other secondary storage devices as they have moving parts e.g. read/write heads.

Characteristics of a Solid State Drive (SSD)

• An SSD drive uses direct access to data (files). There would be no deterioration in read times as there is no physical read-head to move. 

Solid State Drives feature a non-mechanical design of NAND flash mounted on circuit boards

• NAND flash is shock resistant

• SSDs have greater performance

• Computers with SSDs have quicker boot up time

• SSDs do not require defragmentation

• Defragmentation may perform “trim” command which may slightly improve the speed of future write operations

• SSDs consume less power:

• SSDs use significantly less power at peak load than hard drives

• SSDs are lighter / smaller:

• SSDs offer cost savings in the long run for businesses with lower energy usage and greater productivity.

• SSDs require very little power to operate which translates into less heat output by the system.

• With no moving parts, SSDs run at near silent operation.

ISSUES Surrounding the defragmentation of a Solid State Drive

• SSD uses direct access to data (files) so there would be no improvement in read times as there’s no physical read-head to move 

• Defragmentation may perform “trim” command which may slightly improve the speed of future write operations.  It is not recommended to defragment a SSD as it will not improve performance.  It will wear out the electrical components that store the data. 

• SSD is currently made out NAND based flash memory 

• NAND based flash memory has a limited lifespan – defragmentation process may shorten its lifespan. 

Characteristics of an Optical Drive

• Optical drives work by using lasers to store data

• Burning microscopic indentations into a disc such as a CD.

• This pattern of indentations is created in a spiral pattern, starting from the middle.

• Indentations and their absence create pits and lands.

• A laser is aimed at the disc and reflected back, which can cause interference with the original laser.

• DVD-ROM uses the same techniques to store data, but the data is stored on two layers.

• Some optical drives have two lasers of differing wavelength that are used to read data from the two layers.

• On Blu-ray pits and lands are stored closer together, meaning that the laser’s wavelength must be shorter (blue).

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