5.6 - Secondary Storage Devices Flashcards
What does a hard disk consist of?
Consists of a number of circular platters which are made from a magnetic material
Above each platters hovers an actuating arm on which is a read/write head
What is the binary pattern for magnetic storage devices?
Positive or negative polarisation of magnetic particles
Changes from negative to positive or vice versa creates electromagnetic pulses
Each pulse/change odd read as a 1
How can more data be for into the same physical space?
- more densely packed platters
- smaller magnetic parts and read/write heads
How is disk latency created?
- seek delay = time the head takes to move across the disk
- rotational delay = time the disk takes to move to the correct sector underneath the read/write head
- transfer time to move the actual data
What does the actuating arm do?
The actuating arm allows the read/write head (changes the magnetic polarity of parts of the platter) to access all portions of each platter
How data stored in hard disk drives?
Written in concentric tracks each of which is further divided into sectors
How fast does the platter rotate?
Thousands of times per minute which allows for good read and write speeds
What does a solid state drive consist of?
Consist of NAND flash memory cells and a controller that manages the structure of data on the drive
How are the contents of the SSD stored even when there is no power being supplied?
As the NAND flash memory is non-volatile
How is information stored on an SSD?
In memory cells which are formed of floating gate transistors which trap electrical charge to store information
How is data stored on SSD’s?
In pages which combine to form blocks
What do SSD’s do when needing to overwrite data (as unlike HDD’s they cannot)?
The controller must completely erase the entirety of a page before writing the new information to it
What type of information do optical disks store?
Information which can be read optically by a laser
How are pits created in optical disks?
They’re burnt into the disk by a high-power laser which permanently deforms the surface
How is an optical disk formed..?
One continuous track which spirals from the center of the disk to the outside edge
How are the reflections of the optical disk converted into a digital signal of binary?
By a low power laser beam being passed over the flat surface of an optical disk which reflects back onto a photodiode
- when the laser incident on a pit the light form the beam is scatted in different direction rather than reflected back at the photodiode
- the resulting latter of reflection and scatters can be converted into binary
How is a digital created on recordable and rewritable optical disks?
By a dye on the disks surface:
- when’re there is no dye the disk reader’s laser beam is reflected off the optical disks surface
- where there is dye the laser beam is absorbed by the dye and not reflected at all
What type of dye is used by recordable optical disks?
Photosensitive dye that changes from opaque to transparent under a high power laser used to write information to the disk
Unaffected by low-power laser
What type of dye is used by rewritable optical disks?
Phase-change dye:
Can be converted multiple times between transparent and opaque depending on the temperature it is heated to allowing data to be erased and rewritten multiple times
Describe the capacity, read/write speeds, latency, portability, power consumption and suitability of HDD’s?
Capacity: high (500GB - 5TB)
R/W speeds: good speeds (100MB/s)
Latency: High
Portability: bulky, heavy and easily damaged by movement (moving parts)
Power Consumption: high
Suitability: good for desktop, PC’s and servers
Describe the capacity, read/write speeds, latency, portability, power consumption and suitability of SSD’s?
Capacity: relatively low (under 1TB)
R/W speeds: very high
Latency: very low
Portability: lightweight and rarely damaged by movement
Power consumption: low
Suitability: laptops, phones and tablets
Describe the capacity, read/write speeds, latency, portability, power consumption and suitability of optical disk?
Capacity: very low, blu ray has the highest
R/W speeds: relatively low speeds
Latency: high
Portability: very small and lightweight, can be damaged by scratches and dirt
Power consumption: high
Suitability: sharing and distributing small volumes of data
