CS Revised: Secondary, virtual and offline memory

what is secondary storage? (volatile? access?)

non-volatile long-term storage, not directly accessible to the processor

What has to happen before the computer can utilise data/programs in secondary storage?

copied into the computer’s memory and transferred to ram before it gets to the processor

access speed: primary vs. secondary storage

secondary has slower access speed

where is secondary storage generally found?

fixed internally in computer casing

magnetic data storage

electromagnets create fields, set tiny magnetised dots to north or south on the magnetic surface of the disk representing binary 1s and 0s

2 examples of magnetic data storage

hard disk drives and magnetic tape drives

HDDs (hard disk drives)

circular platters coated in magnetic material; data stored in concentric tracks and sectors, making data blocks

how is data accessed in HDDs?

several read/write heads move in and out to find the required data block (moving parts!)

magnetic tape drives

data stored in series along a tape that loops around two wheels in a cassette

what are magnetic tape drives usually used for?

back ups and archiving

NAND

based ICs use millions of floating gate transistors to store data as 1s and 0s

what is the cheapest and most common kind of SSD storage?

NAND

where is NOR-based technology used?

EEPROM; NOR is expensive

SSDs (solid state drives)

grid of electrical cells used to send and retrieve data; grids separated into ‘pages’, where data is stored; pages clumped together to form ‘blocks’

how do SSDs store data?

by flashing it onto the chips

what are HDD and SSD both used for?

storing OS, applications, files and folders

capacity of HDDs and SSDs

several hundred gigabytes or a few terabytes of data

HDD vs SSD cost

HDD cheaper per unit of data stored

HDD vs SSD latency

SSD faster data access

why has SSD got a lower latency than HDD?

HDD needs to speed up before they work properly and needs time for particular data-block to rotate to a read/write head; SSD is random access as there are no moving parts

HDD vs SSD robustness

SSD more reliable and shock resistant due to no moving parts. HDD fragile and moving parts could wear out.

HDD vs SSD longevity

HDD greater longevity over time for read/write functions.

why does SSD have a lower longevity?

limited number of read/writes (eg 20GB of read/writes per day for only 3 years)

HDD vs SSD power consumption

SSDs lower power consumption and run cooler than HDD

why SSDs are better for laptops than HDDs

more robust, lower power consumption, runs cooler, lighter

HDD vs SSD dimensions

SSD small due to small memory ICs

where are HDDs often used?

web servers

why are SSDs less suitable to be used in web servers?

limited longevity

why might SSDs be used in web servers anyway?

servers need to respond quickly to user requests; don’t overheat, more energy efficient, faster latency

why are HDDs often used in web servers?

likely to need a lot of data storage; not likely to be portable so moving parts not a problem; higher longevity; too expensive to switch to SSD

why a computer might need to store data in virtual memory

computer exposed to heavy processing can run out of space in RAM, so OS sections off some secondary storage to use temporarily

explain how a computer stores data in virtual memory

when RAM is full, OS sections off some secondary storage and temporarily uses it to store data.

what kind of data is moved to virtual memory?

data that was sitting in the ram but is not currently being used

what happens when the data in virtual memory is needed again?

has to be moved back to RAM - makes computer run slower

what is offline memory? (volatile?) (access?)

non volatile; not fixed within computer - portable and can be removed easily

examples of offline memory

CDs, DVDs, Blu-Ray discs

what kind of memory is optical media?

offline

optical media

polycarbonate plastic disks coated with aluminium

how is data stored in optical media?

‘burned’ by lasers into pits and lands on the spiral tracks representing 1s and 0s

how is data read from optical media?

lasers shone onto pits and lands; light bounces back differently; optical device (drive) reads the binary from reflected light

how to store more data on optical media

shorter laser wavelength = smaller pits and lands = more can fit on; smaller distance between tracks in spiral = more data stored

advantages of optical media

good data transfer rate, cheap

disadvantages of optical media

reflective surface easily scratched and disk easily broken; requires a separate drive; data has to be burnt/ finalised before being used on another device

rank off-line storage slowest to fastest

CD, DVD, Blu-Ray

CD laser

red laser 780nm

CD disk

single layer

CD stands for

compact disc

CD capacity

6-700MB = 80 mins of music

DVD stands for

digital versatile disc

DVD laser

red laser 650nm

DVD disk

dual layer disk; suffers from birefringence

what is birefringence and where is it an issue

light refraction into two beams giving read errors; problem with DVDs as they are dual layered

DVD capacity

8-9GB = 1.5 hours of video + sound if the disk is dual layered

Blu-Ray laser

blue laser 450nm

Blue-Ray disk

single / dual / multiple layer disks

Blu-Ray storage

25GB = 2.5 hours video + sound at much higher quality; up to 128GB for quad-layer

sneaky W of Blu-Rays

uses built-in encryption, aiding companies to maintain copyright

how is data stored in DVD RAM

concentric tracks and sectors, so there is direct access

where is DVD RAM used

early DVD recorders / early MP3 players

how is data read from DVD RAM

drive has a read/write head so read and write operations can be done at the same time; users can watch a recorded programme while recording another

flash memory examples

SD cards, USB memory sticks / Flash pen drives

what kind of technology is used in flash memory

solid state technology - NAND gates

advantages of flash memory

small = cheap, light and portable; produce little heat and are power efficient

how does flash memory usually connect to a computer

USB ports; SD cards have their own ports

disadvantages of flash memory

easily lost, broken or stolen