Transcript Lecture 6

Storage1… or … Memory?
Both hold data and/or instructions but:
Memory resides in semiconductor chips
ON the motherboard
Storage refers to space that is OFF the
motherboard: typically some kind of disk
drive that supports reading and (usually)
writing.
1. These notes treat the terms storage and secondary
storage as synonymous.
Storage/Memory Compared
• Storage is non-volatile: data is not lost
when power is off. But most memory is
volatile - apart from a small amount of
ROM, data is lost when power is off.
• Secondary storage gives much lower
retrieval speeds than does memory
• Secondary storage has much higher
capacity than memory.
Secondary Storage Media
NB Apart from magnetic tape, all
these media allow random access.
The reading device goes directly to
the required location, without having
to trawl through preceding items.
Hard Disks
Pictures from
www.howstuffworks.com
How Hard Disks Work
• A set of metal disks, mounted above one another, are
coated with a metal oxide then highly polished. Data
is stored on the disk surfaces as magnetised spots
within concentric tracks and wedge shaped sectors.
How Hard Disks Work
• The highly polished disk surfaces offer great
precision. This, and the fact the disks spin at speeds
up to 3,000 inches per second (170 mph) - the
read/write head flying above the disk without touching
it - enables vast amounts of data to be stored
(compared with tape’s much lower storage rates per
square centimetre).
Foreign Matter
Read/Write
Head
Disk
Surface
This diagram shows the
importance of keeping disks
free from dust. The gap
between read/write head
and disk surface is very
small.
Fixed v Removable Drives
• The traditional hard drive is an integral part of the
computer, housed in a disk bay within the case that
also contains the motherboard.
• Jaz and Zip drives are removable, enabling large
amounts of data to be carried from one machine to
another.
(It may be, however, that the relatively large storage
capacities of write-enabled CD’s and the much larger
capacities of DVD - both discussed later - will exert a
“squeeze” on the more expensive removable drives,
which don’t hold more than 2GB at present.)
Hard Disk Statistics
• Capacity - at time of writing a typical
PC hard disk will store 8 to 20 gigabytes
• Speed - two ways of measuring this:
– data rate: modern disk drives deliver 40
megabytes per second to the cpu
– seek time: from cpu requesting a file, to
the first byte of data from that file being
supplied, is now typically 10 milliseconds
or less
Creating More Space
All binary files contain repetition. Compression
software like Winzip can reduce the space
they take up, but there is a performance cost
since the files must now be decompressed
before they can be used.
So before resorting to file compression, be
sure to get rid of unwanted files and archive
old files.
Optimising a Hard Drive
Where possible, files saved to disk are stored in
contiguous memory locations. But as the disk gets
full, new files get chopped up and stored wherever a
few kilobytes of space can be stored. Retrieval times
get slower as more locations have to be searched.
You can run a defragmentation program once a
month. (There is always one in utilities packages like
Norton or the tools that come with Windows.) This will
try to shuffle files into contiguous addresses, so
optimising storage and speeding up access.
Repairing a Drive
Besides the problem of fragmentation,
hard disk errors can also cause
perform-ance to decline.
Disk repair programs are another
example of utilities software. They
search for and repair disk errors, and
should also be run at regular and
frequent intervals - once a month, say.
Virus Protection
• A virus is a program that disrupts your computer.
Symptoms range from irritating messages onscreen, to
total wipe-out of your hard disk.
• You need to check regularly for viruses (especially if
you use the net). Anti virus software is available, to
check for known viruses every time you boot up.
(Two things: one, that anti-virus software will only
check for known viruses, so you need to keep updating
it; two, that it checks on boot-up - so don’t leave your
computer permanently switched on. Boot up once a
day!)
Backing Up
• Virus, disk failure, loss, theft and
damage to floppies - all can cause data
to be lost. Always back up anything
that would cause you grief if lost.
• Floppies are easy to back up and you
should do so at least once a day if they
contain important work - such as an
assignment ….
Tape Drives
• Tape only supports sequential access to data - much
slower than random access. As a backup medium
this is not important - retrieval won’t often be needed.
• Tape drives enable data to be dumped from hard disk
to tape. Like other storage media, they may be inside
the computer or external to it. Usually they’re external, to allow backing up of more than one computer.
• Most tape drives come with their own back up
software to automate the process.
Tape Backup Software
• Can be set to run automatically,
allowing overnight backups while the
computer is not being used.
• Full backups copy all files. Incremental
backups copy only the files that have
been changed since the previous
backup.
• Most backup software compresses data
to maximise storage capacity.
Types of Tape Drive
• QIC (quarter inch cartridge) is still
widely used, storing 4 gigabytes (GB) of
data
• Travan is faster, and stores up to 20 GB
• DAT (digital audio tape) is the fastest
and most expensive. Current versions
can store up to 70 GB
An Alternative
Back-up Strategy
It is increasingly common for companies
who need to back up data to dispense
with in-house methods. Instead, they go
outside the company to rent server
space from specialists in back up
provision.
Floppy Disks
• Are read and written to in the same way
as hard disks, but at lower speeds. The
disk is floppy but, since the old days of
5.25 disks, comes in a rigid plastic
casing
• Are useful for transferring and backing
up small files. (Their use as an archiving
medium is declining due to the much
higher capacity of CD and DVD.)
Looking After Floppy Disks
• Keep ‘em clean!
• Use the writeprotect hole to
prevent accidental
erasure of important
data. It’s the same
principle as on audio
cassette tape.
CD ROM
CDs are made from plastic, with tiny bumps spiralling
out from the centre. These bumps are then covered
with a thin layer of reflective aluminium which is itself
covered with acrylic, on which a label is printed.
Bumps
Reading a CD’s Bumps
The bumps on a CD are tiny: reading them calls for
very precise hardware. A CD drive has 3 basic
elements:
– motor to spin the CD at 200 - 500 rpm
– laser and lens unit to read the bumps
– tracking system to move the laser unit - at micron resolutions and allow the beam to follow the spiral track
There’s no principled difference between music CDs
and data CDs. CD players can’t do anything with a
data CD because they have no means of processing its
information but any PC with CD Drive, speakers and
sound card can play music CDs as well as read data
CDs.
CD Drive Components
CD Speed and Capacity
• Early on, speed was a critical factor, as some
drives read too slowly for quality sound and
visual animation. Now the slowest drives read
fast enough for this.
(You’ll see drives advertised as 4 speed, 8
speed etc. This refers to how many times
faster than the original read-speed of 150 Kb
per second. On most uses, there is no
difference between 24 and 12 speed!)
• CD capacity is either 650MB or 700 MB
CD Writers
• The means of writing (and that includes
copying) CD’s are now easily available.
(Blank disks for CD writers attached to a
PC are much cheaper than those for
use with hi-fi CD copier systems.)
• CD’s are therefore an excellent
archiving and back-up medium for small
to medium sized companies.
DVD ROM
• Digital Versatile (or Video) Disk. Similar
to a CD-ROM but with a capacity of
over 4 MB - and this is likely to rise to
nearer 20 MB.
• DVD ROM disks will hold a two hour
film at better resolution than video tape.
• DVD ROM drives also read CD’s.