Secondary storage Flashcards
What are the characteristics of secondary storage devices
Non-volatile
Slower to access than RAM
Cheaper- has much higher storage capacity
Any type of storage to which contents of RAM are copied
Usually a hard disk, optical or solid state device
What are 3 physical ways of recording 1s and 0s that do not require power
Magnetic storage- by making it behave like a magnet and the north and south poles representing 1s and 0s
eg: hard disk, magnetic tape storage
Optical storage: more reflective parts represent 1s or 0s
eg: CDs, DVDs
Solid-state storage- represents 1s and 0s with little pools of trapped electrons on a microchip
eg: flash memory such as USB memory or SD cards
What does the magnetic secondary storage device consist of
Stacks of disks called platters with a magnetic coating on each surface
Tiny magnetic recording heads on the end of the arm float a millionth of a centimetre above the disk spinning at 110km/h
Data is recorded on each disks along circular tracks, each split into smaller parts called sensors
Outer part of the disk runs faster that the inner part of the disk
How is data read in a hard disk (4)
- Head moves across to be above the right track
- Required sector comes around under the head
- Surface behaving like a magnet causes a tiny current in the head
- The disk controller translates this into 1s and 0s
What is seek time
The time taken to move the heads to the correct track
What is latency
The time taken for the disk to rotate to the correct part of the track
What is disk access time
Time to get data from a disk which includes seek time and latency
What does optical secondary storage (CDs, DVDs, Blue rays) consist of, (reading, wavelength, process)
CD, DVD or Blue ray is made of several layers
Data is written along a single track that moves from the centre of the disk in spiral
CDs and DVDs can be designated R (write once only) or RW (write or read many times)
DVDs use dual layering (2 individual recording layers) which increases storage capacity
CDs and DVDs use red laser and Blue ray uses blue laser
CDs: laser wavelength of 780 nanometres
DVDs: laser wavelength of 650 nanometres
Blue ray: laser wavelength of 405 nanometres
Shorter the wavelength of laser light, greater the storage capacity of medium
When writing, laser heats recording material and non reflective pits on the surface of the disk
If it is written, it has pits in recording layer that are less reflective than flat parts, called lands
When reading, laser reflects differently off the surface, and this is detected by a light sensor
Outer part of the disk runs faster than inner part of the disk so that data passes under the laser at a constant speed
What happens when data is read on a optical storage device
Disk spins in the drive to ensure all data can be read
Tracking mechanism moves the laser into the correct position over the disk
Laser shines on the disk and reflects back onto the sensor
Signals from the sensor are translated into 1s and 0s
What does electrical secondary storage(solid-state storage, flash drives) consist of
Uses chips called NAND flash
Chips made of special type of transistors that can trap electrons in a ‘pool’
Electrons in pools represent 0s, while empty pools represent 1s.
Billions of transistors will fit on a chip, due to this devices are small, light in weight and and have high capacity
It gives much faster access to data than a hard disk due to having no latency
What happens when data is read from a chip in a electrical storage device, then written, and erased
Control signals identify which bit is to be read and apply a small voltage
If electron pool is empty transistor turns on and 1 is read out
if electron pool is full transistor doesn’t turn on and 0 is read out
The control signals are changed to rad other bits
When data is written, control signals identify which bit is to be written and apply a high voltage
This pulls electrons into the pools of those transistors, recording 1s and 0s
Erasing data requires high voltages to remove electrons from the pools. Due to this, both erasing and writing cause transistor to break down slowly
Flash drives can only be rewritten 1 million times before failing
What happens when data is read from a chip in a electrical storage device, then written, and erased
Control signals identify which bit is to be read and apply a small voltage
If electron pool is empty transistor turns on and 1 is read out
if electron pool is full transistor doesn’t turn on and 0 is read out
The control signals are changed to rad other bits
When data is written, control signals identify which bit is to be written and apply a high voltage
This pulls electrons into the pools of those transistors, recording 1s and 0s
Erasing data requires high voltages to remove electrons from the pools. Due to this, both erasing and writing cause transistor to break down slowly
Flash drives can only be rewritten 1 million times before failing
What is cloud storage
A secondary storage, often belonged to a third party, accessed via a network, usually the internet
Flies stored in the cloud can be accessed anywhere using a internet connection
eg: Dropbox, Google drive
It is a example of virtualisation
What is virtualisation?
Any process that hides the true nature of a computing resource, making it look different, usually to simplify the way it is accessed
Advantages of cloud storage
Can access the data from anywhere from any device using a web browser
Data is securely backed up by the company providing the service
No need to transfer your data if you get a new computer
