What is Data Storage?
• ‘Storing’ data means putting the data in a known
place and can later get the data back again from
that place
• ‘Storing’ data = ‘writing’ data or ‘saving’ data
• Getting the data back from its storage location =
‘reading’ data, ‘retrieving’ data or ‘opening’ a file
Primary Storage vs Secondary Storage
• Main Memory (internal memory or primary
storage):
– RAM (random access memory)
– ROM (read only memory)
Primary Storage vs Secondary Storage
• Virtual Memory (VM):
– Use of secondary storage as additional primary memory
– VM is used when the computer has no more available
random access memory (RAM)
– Enable data that is in RAM and not currently being used
to be transferred to the hard disk (HD)
– When the data on the HD is needed
again, swapping occurs (i.e. other
unused data is transferred to the
HD before the original data is
transferred back to RAM)
Primary Storage vs Secondary Storage
• Backing Storage (secondary storage)
– all other data storage devices in a computer,
e.g. hard-drive, etc. which is not directly connected
to the CPU
– Non-volatile (when power is switched off all of the
data in the storage won’t disappear)
– Used to store data for a long time
Backing storage
devices are slower
to access, but can
hold data
permanently...
Factors to consider
• SPEED
• CAPACITY
• COST
• DURABILITY
• PORTABILITY
• RELIABILITY
Storage Media & Devices
• Storage medium: the device that actually holds
the data
• Storage device: the device that saves data onto
the storage medium, or reads data from it
• Sometimes the storage medium is a fixed
(permanent) part of the storage device, e.g. the
magnetic coated discs built into a hard drive
• Sometimes the storage medium is removable from
the device, e.g. a CD-ROM can be taken out of a CD
drive
Accessing Stored Data
• Serial / Sequential Access
– A serial (or sequential) access storage device is one that
stores files one-by-one in a sequence
– A non-computer serial access device is a VHS videotape
• Video is stored on a long piece of tape, when TV shows are recorded
onto the tape, they go on one-by-one, in order...
• If you want to watch a show that you recorded earlier, you have to
rewind / fast-forward through all other shows until you find it
• The shows are only accessible in the same order that you recorded
them
• This type of one-by-one storage and access is called serial access
Accessing Stored Data
• Direct / Random Access
– A direct (or “random”) access storage device is one
that stores files so that they can be instantly
accessed
– No need to search through other
files to get to the one you want
– E.g. a DVD movie
• Unlike the VHS videotape movie,
you can jump to any scene on a DVD
• All parts of the DVD are directly accessible
• This type of file storage is called direct access
Magnetic Storage Devices / Media
• Why Magnetic?
– Magnetic storage media and devices store data in the
form of tiny magnetized dots
– They are created, read and erased using magnetic
fields created by very tiny electromagnets
– E.g. In a magnetic tape, the dots are arranged along the
length of a long plastic strip which has been coated
with a magnetizable layer (audio and video tapes use a
similar technology)
– E.g. In magnetic discs (e.g. floppy disc or
hard-drive), the dots are arranged in
circles on the surface of a plastic, metal
or glass disc that has a magnetizable
coating
Magnetic Storage Devices / Media
• Hard Drives
– Have a very large storage capacity (up to 4TB)
– Can be used to store vast amounts of data
– Hard-drives are random access devices and can be
used to store all types of films, including huge files
such as movies
– Data access speeds are very
fast and is stored inside a hard-
drive on rotating metal or glass
discs (called “platters”)
Magnetic Storage Devices / Media
Hard disks
Direct access
Access arm moves the read/write head on the platters
The drive then rotates the platters to the proper position
All the heads move in and out at the same pace, they are
always positioned on the same track numbers
All the tracks with the same track number form a cylinder
Access Time =
time for head movement
to reach track +
time for half a complete
rotation
Read/write head and access
arm of a hard disk
Magnetic Storage Devices / Media
• Fixed Hard Drive (3.5” in size)
– A hard-drive built into the case of a computer is known
as “fixed”
• Almost every computer has a fixed hard-drive
– Fixed hard-drives act as the
main backing storage device for
almost all computers since they
provide almost instant access to
files (random access and high
access speeds)
– Total storage space =
# readable sides X # cylinders
X sectors per track X bytes per sector
Magnetic Storage Devices / Media
• Portable Hard Drive
– Placed into a small case (2.5” in size) along
with some electronics that allow the hard-drive to
be accessed using a USB or similar connection
– Allow very large amounts of data to be transported
from computer to computer
– Many portable music players (such as the
iPod classic) contain tiny hard-drives
• These miniature devices are just not much
bigger than a stamp, but can still store over
100MB of data!
Magnetic Storage Devices / Media
• Magnetic Tape
– Large capacity, serial access medium
– Accessing individual files on a tape is slow because it
is a serial access medium
– Usages:
• When large amounts of data need to be stored but where
quick access to individual files is not required
– E.g. for data back-up (lots of data but
rarely only accessed in an emergency)
• In some batch-processing applications
– E.g. to hold the list of data that will be
processed
Optical Storage Devices / Media
• Why 'Optical'?
– Optical storage devices save data as patterns of dots (pits)
that can be read using light
• A laser beam is the usual light source
– The data on the storage medium is read by bouncing the
laser beam off the surface of the medium
• If the beam hits a dot it is reflected back
differently to how it would be if there were
no dot
• This difference can be detected so the data
can be read
– Dots can be created using the laser beam
(for media that is writable such as CD-Rs) A magnified view of the
• The beam is used in a high-power mode to dots on the surface of a CD.
actually mark the surface of the medium, The different patterns of
making a dot “burning” data onto a disc dots correspond to the
data stored on the disc.
Optical Storage Devices / Media
• Read-Only Optical Discs
– Read-only optical discs have data written onto
them when they are manufactured
– Data cannot be changed
Optical Storage Devices / Media
• Read-Only Optical Discs
– CD-ROM (Compact Disc - Read-Only Memory)
• Can hold around 700-800MB of data. The data
cannot be altered (non-volatile) so cannot be
accidently deleted
• Are random-access devices
• Used to distribute all sorts of data: software
(e.g. office applications or games), music,
electronic books (e.g. an encyclopedia
with sound and video.)
Optical Storage Devices / Media
• Read-Only Optical Discs
– DVD-ROM (Digital Versatile Disc - Read-Only
Memory)
• Can hold around 4.7GB of data (a dual-layer DVD can
hold twice that)
• Are random-access devices
• Used in the same way as CD-ROMs but
they are also used to store high-quality
video since they can hold more data
Optical Storage Devices / Media
• High Capacity Optical Discs
– Blu-Ray
• A recent replacement for DVDs
• Can hold 25 - 50GB of data (a dual-layer
Blu-Ray disc can hold twice that) 'Blu‘ refers to the fact that the
laser used to read the disc uses
• Are random-access devices blue light instead of red light.
• Used in the same way as DVD-ROMs Blue light has a shorter wave-
but they are also used to store very length than red light (used with
CDs and DVDs).
high-quality, high-definition (HD) Using a blue laser allows more
video since they can hold more data data to be placed closer
together on a Blu-Ray disc so
Blu-Ray has a much higher
storage capacity than these
older discs.
Solid-State Storage Devices
• 'Solid-State'?
– “Solid-state” essentially means “non-mechanical”
– Solid-state storage devices are based on electronic
circuits with no moving parts (no reels of tape, no
spinning discs, no laser beams, etc.)
Solid-state storage devices store data using a
special type of memory called flash memory /
flash drive...
Solid-State Storage Devices
• Flash Memory
– Type of Electronically-Erasable Programmable
Read-Only Memory (EEPROM)
– Non-volatile (like ROM) but the data stored in it
can also be erased or changed (like RAM)
– Can be found in many data storage devices...
Although flash memory is non-volatile, we don’t use it to replace RAM. This is because saving
data to flash memory is very slow compared to saving it to RAM. If a computer were to use flash
memory as a replacement for RAM it would run very slow.
Some portable computers are starting to use flash memory (in the form of solid-state ‘discs’ as a
replacement for hard-drives). No moving parts mean less to go wrong and longer battery life.
Solid-State Storage Devices
• USB Memory Sticks (or ‘thumb-drives’)
– Make many other forms of portable storage almost
obsolete (why burn a CD or DVD when you can more
easily copy your files onto a memory stick?)
– Non-volatile, random-access storage devices
– Has some flash memory connected to a USB interface
• Plug it into your computer and it appears as a drive
• Can then add files, erase files, etc.
• Can use it to move any type of file between computers
– Become much cheaper
in recent years
Solid-State Storage Devices
• Memory Cards
– Provide a compact and non-volatile data storage for
many of our digital devices (cameras, mobile phones,
MP3 players, etc.)
– Come in a variety of shapes and sizes
– Mobile phones contain a Subscriber Identity Module
(SIM) card that contains the phone's number, the
phonebook numbers, text messages, etc.
– Many phones also have extra memory cards to store
music, video, photos, etc.
(e.g Tiny Micro-SD cards)
Cloud Storage
• Where data is stored on multiple servers in a
remote location
• Servers use magnetic and now increasingly solid-
state storage due to the drop in cost
• Generally operates though a web-based API
• Advantages:
– Versions of files are kept (so durable) but the server may
not be reliable
– Files can be accessed from anywhere on the internet
(portable)
– May be cheaper for large companies (reduced energy
cost, like bulk purchase) but more expensive for smaller
group of users
