Understand cloud storage

Topic 3.3 – Cloud Storage

1. Definition & purpose [Syllabus 3.3 (1)]

Cloud storage is a model for storing digital data on remote servers that are owned, maintained and managed by a third‑party provider. Users access their files over the Internet using a web browser, desktop client or API, allowing upload, download and organisation from any device with an Internet connection.

2. How cloud storage works [Syllabus 3.3 (2)]

The service can be described in three stages:

1. Upload: Data is transferred from the user’s device to the provider’s data centre using secure protocols (e.g., HTTPS, SFTP, or TLS‑protected TCP).
2. Replication: The provider creates multiple copies of the data in different physical locations (data centres). This provides fault tolerance, high availability and protection against loss of a single site.
3. Access (download): The user retrieves the required files on demand via a web interface, desktop client or programmatic API.

3. Key features of cloud storage [Syllabus 3.3 (3)]

• Scalability – capacity can be increased or decreased instantly without purchasing new hardware.
• Pay‑as‑you‑go pricing – charges are based on the amount of data stored (usually per GiB per month) and the bandwidth used. Exam questions often require a simple cost calculation using the given rate.
• High availability – redundant servers aim for “five‑nines” (99.999 %) uptime.
• Automatic backup & versioning – earlier versions of a file can be restored, reducing the impact of accidental deletion.
• Global accessibility – files can be accessed from any location with Internet connectivity.
• Data compression (optional) – many services automatically apply lossless compression to reduce storage space and bandwidth; some also support lossy compression for media files.

4. Advantages & disadvantages compared with local secondary storage [Syllabus 3.3 (4)]

Advantages (vs. local secondary storage)	Disadvantages (vs. local secondary storage)
Reduces need for physical media – no bulky external drives.	Requires an Internet connection; no access when offline.
Easy collaboration and sharing of files across locations.	Potential security and privacy concerns with third‑party providers.
Automatic updates, maintenance and backup handled by the provider.	Ongoing subscription or pay‑per‑use costs.
Scalable storage – pay only for what you use.	Data‑sovereignty issues – data may be stored in jurisdictions with different laws.
Built‑in compression can reduce required storage space.	Performance can be slower than a local SSD for large, frequent transfers.

5. Security considerations [Syllabus 3.3 (5)]

To protect data against common cyber‑security threats, providers and users should implement a layered approach:

Security measure	What it protects	Typical threat mitigated
Encryption in transit (TLS/SSL) – a protocol suite that encrypts data while it travels over the network.	Prevents eavesdropping and data‑sniffing.	Sniffing / Man‑in‑the‑middle attacks.
Encryption at rest (AES‑256) – a symmetric‑key algorithm used to encrypt stored files.	Protects data if a server or storage device is compromised.	Physical theft, unauthorised server access.
Strong authentication (e.g., two‑factor authentication, 2FA)	Ensures only the legitimate user can log in.	Brute‑force attacks, credential theft.
Access control lists (ACLs) & permission settings	Limits who can view, edit or delete each file.	Phishing‑derived unauthorised access, insider misuse.
Regular audits & compliance (ISO 27001, GDPR, HIPAA)	Demonstrates that the provider follows recognised security and legal standards.	Regulatory breaches, lack of accountability.

Evaluation prompt (AO3): Which of the above measures would be most effective against a phishing attack, and why?

Common misconceptions [Syllabus 3.3 (5‑c)]

• “Cloud = backup” – cloud storage is primarily for everyday file access; a separate backup strategy is still recommended.
• “Encryption guarantees privacy” – only if encryption keys are managed securely; loss of keys makes data unrecoverable.
• “All data is automatically replicated” – the level of replication varies between providers and service tiers.

6. Data compression in cloud storage [Syllabus 3.3 (6)]

• Lossless compression (e.g., ZIP, PNG) – reduces file size without any loss of information; ideal for documents and software.
• Lossy compression (e.g., JPEG, MP3) – discards some data to achieve greater size reduction; acceptable for photos, video and audio where perfect fidelity is not required.
• Many cloud services automatically apply lossless compression to uploaded files, which can lower storage costs and bandwidth usage.

7. Virtual memory – a brief link [Syllabus 3.3 (7)]

Virtual memory extends a computer’s RAM by using secondary storage (often a hard‑disc or SSD). In a cloud‑computing context, remote storage can act as a further extension, enabling “cloud‑based virtual memory” for virtual machines. While not part of the core cloud‑storage definition, the concept illustrates the hierarchy of storage from registers → RAM → local secondary storage → remote cloud storage.

8. Common cloud‑storage services [Syllabus 3.3 (8)]

• Google Drive – integrates with Google Workspace; offers 15 GiB free, paid tiers up to 2 TiB.
• Microsoft OneDrive – bundled with Office 365; personal free tier 5 GiB.
• Dropbox – strong sync engine; free tier 2 GiB.
• Amazon S3 (Simple Storage Service) – object storage for developers; pricing per GiB‑month and per‑request.
• iCloud – Apple ecosystem; free 5 GiB, paid upgrades.

9. Calculating storage requirements (binary prefixes) [Syllabus 3.3 (9)]

Cambridge exams use binary prefixes where 1 KiB = 1024 bytes, 1 MiB = 1024 KiB and 1 GiB = 1024 MiB.

Formula

$$\text{Total required space} = \sum_{i=1}^{n} \text{size of file}_i$$

Worked example

If a student has 150 video clips each averaging 4 MiB:

$$150 \times 4\text{ MiB} = 600\text{ MiB}$$

Convert to GiB (use 1024 for conversion):

$$600\text{ MiB} \div 1024 = 0.5859\text{ GiB} \approx 0.59\text{ GiB}$$

Cost calculation example (exam style)

A cloud service charges $0.02 per GiB per month. Storing the 0.59 GiB for 3 months costs:

$$0.59\text{ GiB} \times \$0.02/\text{GiB·month} \times 3\text{ months} = \$0.0354 \approx \$0.04$$

10. Cross‑topic links [Syllabus 3.3 (10)]

• Data transmission (Topic 2.2) – Cloud storage relies on packet‑switched networks, Wi‑Fi or mobile data to move files.
• Encryption (Topic 2.3) – The same algorithms (AES, RSA, TLS) protect data in transit and at rest.
• Error detection & correction (Topic 2.1) – Protocols such as TCP use checksums and retransmission to ensure data arrives intact.
• Internet basics (Topic 2.4) – DNS resolves the provider’s domain name to the IP address of the data centre.
• Virtual memory & storage hierarchy (Topic 1.5) – Cloud storage sits at the top of the hierarchy, extending local secondary storage.
• Data compression (Topic 1.3) – Reduces the amount of data that must be transferred and stored.

11. Suggested diagram [Syllabus 3.3 (11)]

12. Practice questions (AO2 & AO3) [Syllabus 3.3 (12)]

1. Explain two advantages and two disadvantages of using cloud storage for a school computer lab, explicitly comparing it with local secondary storage (e.g., external hard‑discs).
2. A student uploads a 2 GiB video file to a cloud service that charges $0.02 per GiB per month. How much will the storage cost after 3 months? Show your calculation.
3. Describe how data replication improves reliability in cloud storage. Include a brief mention of the effect of a single data‑centre failure.
4. List three security measures that should be implemented when using cloud storage for sensitive data. For each measure, state which cyber‑security threat it helps to mitigate.
5. Evaluation (AO3): A school is considering moving its exam papers to a cloud service. Discuss the security benefits and the possible risks, and recommend one additional safeguard the school could adopt.
6. Explain the difference between lossless and lossy compression. Give an example of a file type that would typically use each method before being stored in the cloud.
7. Calculate the total storage needed (in GiB) for 25 documents of 850 KiB each and 8 photos of 3.2 MiB each. Show your working.

13. Summary [Syllabus 3.3 (13)]

Cloud storage provides flexible, scalable, and globally accessible data storage by hosting files on remote servers. Understanding the upload‑replication‑download process, the key features (including pay‑as‑you‑go pricing and optional compression), the advantages and disadvantages compared with local secondary storage, and the layered security measures required enables students to evaluate when cloud services are appropriate and to use them safely in personal, educational or professional contexts.