Ch.4: Storage Devices
Theory Notes
4.1 Backing storage
Definition and characteristics:
• Non-volatile storage
• Stores data permanently
• Not directly accessed by the CPU, so it’s slower to access than the main memory
Purpose:
•
•
•
For permanent storage of files and software
To store data that is not currently required by the CPU
To store data to transfer it to another computer
The following table shows the differences between Main memory and backing storage:
Main/Internal memory
Backing storage
Directly accessible by CPU
Not directly accessible by CPU
Faster access speed
Slower access speed
Tends to have greater storage
RAM and ROM are fixed inside the computer
Backing storage can either be fixed or it can be
removable
Can be volatile (RAM) and non-volatile (ROM)
Non-volatile
More expensive per unit storage
Cheaper per unit storage
Stores start-up instructions permanently (ROM)
and stores data, instructions, software during
processing (RAM)
Stores files and software permanently
Factors to consider when comparing backing storage devices:
• Storage capacity
• Data transfer rate
• Data access time
• Durability
• Portability
• Price per GB
Data transfer rate: Rate at which data can be sent from a storage device to a computer (or vice versa).
Data access time: Time it takes to locate specific data stored on a storage media.
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
4.2 Storage media vs. Storage device
Storage Media definition:
Hardware on which the data is stored (for example, CD and DVD).
Storage Device definition:
Hardware used to read from or write to the media (for example, a CD/DVD reader or writer).
4.3 Magnetic media and magnetic storage devices
Fixed Hard Disk Drive (HDD)
Advantages:
• Large storage capacity
• Fast data transfer rate
• Fast data access times
• Has great read/write longevity
Disadvantages:
• An incorrect shutdown procedure could lead to loss of data
• They have many moving parts, which can affect their overall reliability
• Quite noisy when compared to SSDs
Uses:
• To store the operating system
• To store system software and working files
• In real-time systems (systems that need fast retrieval and storage of data)
• Used in file servers for computer networks
Portable Hard Disk Drive
Advantages:
• Large storage capacity
• Fast data transfer rate
• Fast data access times
• Portable, so it can be used in file transfer
Disadvantage:
• An incorrect shutdown procedure could lead to loss of data
• They have many moving parts, which can affect their overall reliability
• Data transfer rate is not as fast as for fixed hard drives
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
Uses:
• To store backups
• To transfer data between computers
Magnetic Tape Drive
Advantages:
• Huge storage capacity
• It is a very robust technology (lasts for a long time)
• Fast data transfer rate
• Cheaper per unit memory than HDD
Disadvantages:
• Very slow data access times
-
Data is accessed serially; this means files are accessed one by one
• They are affected by magnetic fields
Uses:
• To store backups
• Used in long-term archiving of data
Magnetic media example: Magnetic tape
Magnetic devices examples: Hard disk drive, Portable hard disk drive, Magnetic tape drive
4.3.1 Optical media and optical storage devices
Advantages of optical media:
•
•
Cheaper to buy than other storage devices (as they tend to have small storage space)
Portable
Disadvantages of optical media:
•
•
•
•
Tends to have lower storage space than other types of storage devices
Slow data transfer rate
Slow access times
Not compatible with all computers
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
CDs and DVDs
CDs and DVDs can be categorized as follows:
• ROM – can only be read
• R – write only once
• RW – can be written to or read from many times
CD-ROM and DVD-ROM
Advantage:
• Prevents the deletion or overwriting of important data
Disadvantages:
• Data transfer rate is slower than for hard disks
• Data access times are slower than for hard disks
Uses:
• Used in applications where there is a need to prevent the deletion or overwriting of
important data
• CD-ROMs are used to store music, software, and computer games
• DVD-ROMs are used to store movies and more sophisticated computer games
CD-R and DVD-R
Advantages:
• Cheaper to buy than RW disks for the same storage
• Once burned, they behave like ROM, which protects against accidental
overwriting of data
Burning refers to adding data to an optical device.
Disadvantages:
• Can only be recorded once; if an error occurs, then the disk must be thrown away
• Not all CD/DVD drives can read them
Uses:
• Archiving of data
• File transfer between different computers
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
CD-RW and DVD-RW
Advantages:
• Can be written over many times
• Not as wasteful as R format as the files/data can be added at a later stage
Disadvantages:
• More expensive to buy than ROM and R formats for the same storage
• It is possible to accidentally overwrite data
Uses:
• To store data and backups
• File transfer between different computers
• Used in CCTV systems
DVD-RAM
Main Advantage:
• Can read and write data at the same time
• Has greater storage capacity than CDs
Main Disadvantage:
• More expensive to buy than other CDs (and DVDs for the same storage)
Uses:
• To store backups
• File transfer between different computers
Blu-ray disc
Advantages:
• Greater storage capacity than other optical media
• Faster data transfer rate than other optical media
• Faster data access times than other optical media
• Automatically comes with a secure encryption system that helps to prevent
piracy and copyright infringement
Disadvantage:
• The most expensive optical media to buy
Uses:
• To store HD movies and games
• To store backups
• File transfer between different computers
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
We can summarize the differences between different optical media in the following
diagram:
CD
DVD
DVD-RAM
Blu-ray disc
Greater storage capacity
Faster data transfer rate
Faster data access times
More expensive to buy
Optical media examples: CD, DVD, Blu-ray disc
Optical devices examples: CD drive, DVD drive, Blu-ray drive
4.3.2 Solid-state media and solid-state storage devices
Solid State Drives (SSDs)
Advantages of SSDs over HDDs:
•
•
•
•
•
•
•
•
Faster access times
Faster data transfer rate
No moving parts, so more reliable
Less power consumption
They run much cooler
They occupy less physical space
They run quieter
They are lighter
Disadvantages of SSDs over HDDs:
• More expensive per unit memory
• HDDs have greater longevity than SSDs
Uses:
• To store the operating system
• To store system software and working files
• In real-time systems (systems that need fast retrieval and storage of data)
• Used in smartphones and tablets as the main backing storage device
Any device that uses solid-state technology can be referred to as a flash drive.
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
Pen drive/Memory stick
Advantages:
• Very portable and compact
• Very robust
• Not affected by magnetic fields
Disadvantages:
• Easy to lose
• Lower storage capacity compared to HDDs
• More expensive per unit memory than HDDs
Uses:
• To store backups
• File transfer between different computers
Memory cards
Examples:
• SD cards (secure digital card)
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Used in digital cameras, audio players, smartphones and tablets
• XD cards (extreme digital card)
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Used in digital cameras
• CFast card (compact-fast card)
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Used in expensive digital and video cameras
Advantages:
• Small and can be easily removed and used in another device
• Very durable
Disadvantages:
• More expensive per unit memory than HDDs
• Have a lower storage capacity than HDDs
• Not all computers come with memory card readers built-in
Uses:
• To store photos on a digital camera
• Used as mobile phone memory cards
• To store backups in hand-held devices
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
Error messages that may appear when attempting to save the work on memory cards:
• Device not recognized
• Corrupt card
• Medium is full
• Write error
• Virus found on the card
Solid-state media example: Memory cards (SD card, XD card, CFast card)
Solid-state devices examples: SSD, Flash drive, Pen drive, Memory card reader
The following table summarizes the comparison between different types of storage:
Magnetic Storage
Optical Storage
Solid-State Storage
Highest
Low
High
Data Access
Speed
Fast (Except for
Magnetic Tape Drive)
Slow
Fastest
Data Transfer
Rate
Fast
Slow
Fastest
Durability
Vulnerable to shocks
and magnetic fields
More durable/robust
than magnetic storage
More durable/robust than
optical and magnetic storage
Price Per GB
Low
Low
High
Special
Characteristics
Great longevity
Not compatible with all
devices
Storage
Capacity
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•
•
•
Runs cool
Low power consumption
Runs quietly
Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
4.4 Cloud storage (Cloud computing)
Definition:
• Online storage platform
• Data is stored in a remote physical location ..
• .. using hundreds of interlinked data servers
A server is a computer.
Data servers used to store data are referred to as Cloud.
How data is stored using cloud storage:
• User accesses the cloud storage using the internet ..
• .. from any device
• User uploads files to a data server
• Users pay a monthly/annual fee for storage used
How data is managed using cloud storage:
• The server is managed by a cloud provider
• Data is automatically backed up
• If one server fails, there are others used as backup
Steps to transfer files from a computer at home to another computer at university using cloud
storage:
1 User logs into the cloud from any device at home using the internet
2 User ensures sufficient storage in the cloud account
3 User uploads the files from his home computer to the data server
4 User then logs into the cloud storage account from the university’s computer using the
internet
5 Finally, the user downloads the files on the university’s computer
Advantages compared to storing data locally:
• Data is accessible at any time, from any device, anywhere in the world, provided internet
access is available
• More available storage space
• Storage capacity can be increased without adding additional physical devices
• Automatic backups can be made
• Users only pay for the storage they use
• No need to carry any storage device, so data will not be lost if the storage device is
lost/stolen
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Eng. Mustafa El-Komy
Ch.4: Storage Devices
Theory Notes
Disadvantages compared to storing data locally:
• May lose access to data if internet connection is lost/not available
• Users must have a reliable internet connection to store data
• Could incur an ongoing cost
• Reliant on the cloud provider to maintain the hardware
• Loss of control over the storage of data (storage is controlled by the cloud provider)
• May be less secure as multiple copies of the data are stored for a long period of time
Last two disadvantages are issues related to the security of data in the cloud.
Eng. Omar El Safty
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Eng. Mustafa El-Komy