Data Storage

Data Storage Overview

●​ Purpose: Data is stored for processing and later retrieval.

Types of Storage
●​ Primary Storage: Known as ‘primary memory’ or ‘memory’. Data is stored here

for quick access by the computer's processor.

●​ Secondary Storage: Used for longer-term storage, typically larger capacity

than primary storage.

●​ Offline Storage: Storage that is not immediately accessible, often used for
backups or archival purposes.

Primary Storage
●​ Function: Holds data ready for processing by the processor.

●​ Capacity: Smaller than secondary or offline storage.

●​ Types:

○​ Random Access Memory (RAM): Holds data to be processed and active

programs.

○​ Read Only Memory (ROM): Stores non-volatile data (data remains even
when the computer is powered off).

Random Access Memory (RAM)

●​ Function: Stores data and programs that are in use.

●​ Volatility: RAM is volatile, meaning data is erased when power is turned off.
 ●​ Expansion: RAM can be expanded to increase storage capacity.
●​ Types of Devices:
○​ Embedded Computers: Such as calculators or digital watches with

limited RAM.

○​ Desktops, Laptops, Tablets, Smartphones: Require more RAM for

operating systems and active programs.

○​ Printers: Store data in RAM before printing.

Types of RAM
1.​ Dynamic RAM (DRAM)

○​ Structure: Made up of transistors and capacitors.

○​ Function: Capacitors hold bits of information, while transistors act as

switches to read or change data.

○​ Refreshing: Needs constant refreshing every 15 microseconds to

prevent data loss.

○​ Advantages:
■​ Less expensive than SRAM.

■​ Consumes less power.

■​ Has higher storage capacity.

2.​ Static RAM (SRAM)

○​ Structure: Uses ‘flip-flops’ to hold each bit of memory.

○​ No Refreshing: Does not need constant refreshing.

○​ Speed: Faster than DRAM (25 nanoseconds vs. 60 nanoseconds for

DRAM).

○​ Use: Preferred in situations requiring high speed, such as processor

memory caches.
Read Only Memory (ROM)
●​ Function: Data in ROM can be read but not written to.

●​ Non-Volatility: Unlike RAM, ROM does not lose its contents when the power is

turned off.

●​ Types of ROM:
○​ Firmware: Used in embedded devices to run the device.

○​ Bootstrap: Holds basic hardware structure instructions to load the

operating system in general-purpose computers.

○​ Erasable Programmable ROM (EPROM): Can be erased and rewritten,

used for firmware upgrades (e.g., in smart TVs, Blu-ray players).

Applications of ROM and RAM

●​ ROM:

○​ Stores factory settings, such as remote control frequencies.

○​ Holds start-up routines and control instructions for devices.

●​ RAM:
○​ Stores user-generated routines and data/instructions.

○​ Holds data for processing in general-purpose computers.

RAM vs ROM Comparison

Feature RAM ROM

Function Temporary data storage Permanent data storage

Volatility Volatile (data lost when powered Non-volatile (data retained when

off) powered off)

Capacity Can be expanded, varying in size Typically smaller in capacity

Content Can be written to, erased, and Typically read-only, can be

modified programmed once

Use Holds data for processing and Stores firmware and bootstraps

active programs

Secondary Storage

Overview

●​ Secondary Storage: Refers to storage devices not directly addressable by the

CPU, non-volatile, and used for long-term data storage.

●​ Capacity: Can store more data than primary storage (e.g., 500 MB to several
terabytes).

●​ Data Access: Slower than RAM or ROM, but essential for storing applications,
OS, device drivers, and general files (documents, photos, music).

Types of Secondary Storage

1.​ Hard Disk Drives (HDD)

○​ Storage: Uses magnetic surfaces of disks (platters) to store data in

digital format.

○​ Structure: Multiple platters that spin at about 7000 rotations per

second.

○​ Read/Write Heads: Move quickly to access data on the platters, typically

50 times per second.
 ○​ Data Access: Data stored in tracks on the platters, with sectors
allocated for storage. Latency occurs when waiting for a specific data

block to rotate under the read/write head.

○​ Fragmentation: Data becomes fragmented over time due to file deletions

and editing. Defragmentation software helps organize sectors for faster

access.

2.​ Solid-State Drives (SSD)

○​ No Moving Parts: Stores data in millions of tiny transistors within the

chip, making it faster and more reliable than HDDs.

○​ Structure: Cells and transistors arranged in a grid, with NOR and NAND
flash structures.

○​ Floating Gate & Control Gate: The control gate regulates current flow,
while the floating gate stores electrical charge to represent binary

values (0 or 1).

○​ Advantages:
■​ More reliable due to no moving parts.

■​ Faster data access.

■​ Lower power consumption and heat generation.

■​ More suitable for portable devices like laptops.

3.​ Memory Sticks/Flash Memories

○​ Uses: Small, lightweight devices for transferring files between

computers and as backup devices for photos or music.

○​ Application: Can serve as a dongle for complex software, preventing

unauthorized use or software copying.
Comparison of HDD vs. SSD

Feature HDD SSD

Power More power consumption Less power consumption
Consumption
Cost Cheaper More expensive

Storage Typically less than 1TB for Min 500GB and max 2TB for
Capacity
notebooks, max 4TB for notebooks, up to 10TB for

desktops desktops

Movement Moving parts, can hear clicks No moving parts, silent

and spinning operation

Vibrations Can generate vibrations due to No vibrations

spinning platter

Heat Generates more heat Generates less heat

Generation
File Opening Slower Faster
Speed
Magnetic Effect Data can be erased by Safe from magnetic effects

magnets

Benefits and Drawbacks of HDD & SSD

●​ HDD Benefits:
○​ Can store vast amounts of data at an affordable price.

○​ Reliable and widely used technology.

○​ Small in size but large in capacity.

 ●​ HDD Drawbacks:
○​ Moving parts wear out over time.

○​ Slower performance compared to SSD.

○​ More fragile and less robust.

○​ Higher power consumption.

●​ SSD Benefits:
○​ No moving parts, making it more reliable.

○​ Lighter, ideal for laptops and portable devices.

○​ Faster data access speeds.

○​ Lower power consumption and heat generation.

●​ SSD Drawbacks:
○​ Limited lifespan (endurance): Only 20GB of write operations per day

over three years.

○​ Not suitable for servers with high write operations.

Primary Storage vs Secondary Storage

Feature Primary Storage Secondary Storage

Access Directly accessible by the Not directly accessible by the

processor processor

Data Temporarily stores data Permanently stores data until

erased

Speed Instant read/write operations Slower data transfer to RAM

Capacity A few gigabytes Up to terabytes or more

Removability Fixed within the computer Some devices are removable

Secondary Storage

There are three types of secondary storage:

1.​ Magnetic Storage

2.​ Optical Storage

3.​ Solid-State Storage

Each works differently and is used for different purposes.

Magnetic Storage

🔹 How it Works:
●​ Uses electromagnets and magnetic fields to control tiny magnetic dots

representing binary data.

●​ Devices can be tape-based or disk-based.

🔹 Tape-Based Devices:
●​ Use a cartridge of looped magnetic tape.

●​ Magnetized data dots are stored in series along the tape.

●​ Data is read/written using an electromagnetic read/write head.

●​ Capacity: Up to 185 TB.

🔹 Disk-Based Devices:
●​ Hard disk drives (HDD) contain several metal/glass platters.

●​ Most common secondary storage device.

●​ Capacity: Up to 8 TB.
●​ Transfer Speed: Faster than tape-based devices.
Optical Storage

🔹 How it Works:
●​ Reads data using a laser beam on plastic disks coated with reflective

aluminum.

●​ Data is stored in pits and lands (representing binary).

●​ Process of writing data = "burning".
🔹 Optical Storage Types:
Storage Type Laser Color Wavelength Data Layer Track
Pitch
CD Red 780 nm Single (1.2mm) 1.60 µm

DVD Red 650 nm Dual (0.6mm 0.74 µm

each)

Blu-ray Blue 405 nm Single (1.1mm) 0.30 µm

🔹 CD/DVD Storage Formats:

●​ ROM – Read-only (pre-burned data).
●​ -R / +R – Write once.
●​ -RW / +RW – Can be rewritten multiple times.
🔹 DVD-RAM:
●​ Uses concentric tracks (not spiral).
●​ Allows simultaneous read/write operations.

●​ Lifespan: Over 30 years (good for archiving).

🔹 Blu-ray Disks:
●​ Use a blue-violet laser (shorter wavelength).
 ●​ Stores 5x more data than a DVD.
●​ Includes encryption to prevent piracy.

Solid-State Storage

🔹 Characteristics:
●​ No moving parts → Very fast & durable.
●​ Used in USB drives, SSDs, and SD cards.

Device Capacity Transfer Portability Durability Cost per MB

Speed
USB RAM Up to 1TB Fast Yes Fairly durable (but Cheap
Stick
(180MB/s) small & easy to

lose)

Solid-State Up to 4TB Very fast Yes Highly durable Expensive

Drive (SSD)
(230MB/s)

SD Card Up to 2TB Medium Yes Fairly durable Cheap

(48MB/s)

Choosing an Appropriate Secondary Storage Device

🔹 Key Factors:
Factor Description
Capacity How much data the medium can store.

Transfer How quickly data can be read/written.

Speed
Portability How easily it can be moved between

locations.

Durability Resistance to damage and wear.

Cost Measured as cost per MB/GB.

🔹 Comparison of Storage Devices:

Format/Device Capacity Transfer Portable Durability Cost per MB
Speed
HDD Up to Fast Yes Very durable Very cheap
8TB (128MB/s)

Magnetic Tape Up to Medium Yes Durable case but Extremely

fragile tape cheap
185TB (30MB/s)

CD 700MB Slow Yes Easily Expensive for

scratched/broken large data
(10MB/s)

DVD 4.7 – Medium Yes Easily Expensive for

scratched/broken large data
9.4GB (33MB/s)

Blu-ray 25 – Fast Yes Easily Expensive for

scratched/broken large data
128GB (72MB/s)

Secondary Storage for Different Situations

Situation Storage Medium Reasons

Storing OS, programs, HDD / SSD Large capacity, high transfer
and data speed, rewritable
Software distribution CD-ROM / DVD-ROM Cheap, portable, read-only
 Movie distribution DVD-ROM / Blu-ray Large capacity, portable,
read-only
Transferring data USB Stick / SD Card Cheap, portable
Data backup Magnetic Tape / Large capacity, removable,
rewritable
Portable HDD

Archiving data CD-R / DVD-R / Long-term storage, data cannot

be erased
Blu-ray R

Virtual Memory

Definition:
●​ Virtual Memory (VM) is a portion of HDD or SSD used as an extension of RAM

for temporary storage.

How it Works:
●​ Created by partitioning a section of the hard drive.

●​ Used when RAM is full to prevent system crashes.

●​ Allows systems to run programs exceeding physical memory limits.

Key Concept:
●​ RAM = Physical Memory

●​ Virtual Memory = RAM + Swap Space (on HDD/SSD)

Benefits:​
✔ Programs larger than physical memory can still execute.​

✔ Prevents wasting memory on unused data.​

✔ Reduces the need for expensive RAM upgrades.

Drawbacks:​
✖ Disk Thrashing: Excessive swapping of data between RAM and disk slows system

performance.​

✖ Thrash Point: System halts due to excessive memory swapping instead of actual
execution.

Cloud Storage

Definition:
●​ Data is stored on remote servers instead of local devices.

●​ Data redundancy ensures availability even during maintenance or failure.

Types:
1.​ Public Cloud – Storage provided by an external company.
2.​ Private Cloud – Dedicated storage within a company’s internal system.

3.​ Hybrid Cloud – Combination of public and private cloud storage.

Benefits:​
✔ Accessible anytime, anywhere with the internet.​

✔ No need for physical storage devices.​

✔ Provides remote backup, preventing data loss.​

✔ Almost unlimited storage capacity.

Drawbacks:​
✖ Internet Dependency: Slow or unstable connections hinder access.​

✖ High Costs: Large storage and high data transfers can be expensive.​

✖ Cloud Provider Risks: Service failure or shutdown could result in data loss.

Potential Data Loss & Security Concerns

Risks:
●​ XEN security threat forced mass server reboots.

●​ Cloud provider data loss during backup failures.

Security Considerations:​
​ ✔ Physical security of data centers.​

​ ✔ Protection against natural disasters or power failures.​

​ ✔ Employee access restrictions to prevent data misuse.