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DSE ICT | Cloud Computing and Virtualisation Lesson-Prep Guide (Tutor Edition) | IaaS, PaaS and SaaS Lines, Host and Cost Arithmetic, SLA Downtime
Students say they will move to the cloud but cannot say how many hosts, what it costs, or how many minutes a year it will be down. This lesson makes cloud a decision.
Objectives — Students can: (1) compute the hosts needed from VM count and consolidation ratio, and compare on-premises with cloud monthly cost; (2) separate the IaaS, PaaS and SaaS responsibility lines with a school example each; (3) convert an SLA into monthly downtime and explain why chained services multiply availability.
01 Prerequisites (3-minute check)
If any of these fails, fix it before the lesson starts.
| Skill | Quick check | Pass mark |
|---|---|---|
| Binaries and storage | How many GB in one TB | 1,024 GB |
| Percentages | What 99.9% costs in downtime | 43.2 minutes a month |
| Cost types | Fixed against variable cost | Own kit is fixed, cloud is variable |
02 Lesson flow (60 minutes)
| Time | Stage | Tutor does | Student does |
|---|---|---|---|
| 0-10 min | What virtualisation is | A hypervisor slices one host into VMs | Explain overcommit |
| 10-24 min | Host count | VMs divided by density, rounded up | Compute 7 hosts |
| 24-36 min | Monthly cost | Depreciation plus power against on-demand | 8,330.56 against 10,400 |
| 36-46 min | Three layers | Where each model stops | One example each |
| 46-56 min | Availability | SLA into minutes, chained service multiplied | Compute 259 minutes |
| 56-60 min | Exit check | Three questions marked on the spot | All three right |
03 Teaching script (read it out)
Scene 1: Count the hosts before you talk money
Forty VMs at 2 vCPU and 8 GB each, on a 16-core, 128 GB host, with a conservative density of six VMs per host, need 7 hosts (40 divided by 6 is 6.67, rounded up).
On premises: HK$48,000 per host over four years is HK$1,000 a month in depreciation; at 220 W a host draws 158.4 kWh a month, HK$190.08 at HK$1.2 per unit, so seven hosts cost HK$8,330.56. Cloud at HK$260 per VM is HK$10,400, HK$2,069.44 more, or 24.8%.
But cloud scales in minutes while bought hosts must be paid for four years. The strong answer is a mix: steady load on site, exam-season peaks in the cloud.
On premises: HK$48,000 per host over four years is HK$1,000 a month in depreciation; at 220 W a host draws 158.4 kWh a month, HK$190.08 at HK$1.2 per unit, so seven hosts cost HK$8,330.56. Cloud at HK$260 per VM is HK$10,400, HK$2,069.44 more, or 24.8%.
But cloud scales in minutes while bought hosts must be paid for four years. The strong answer is a mix: steady load on site, exam-season peaks in the cloud.
Ask students: why the host count rounds up rather than to the nearest whole number (half a VM is not a thing).
Scene 2: The three layers differ by responsibility, not by technology
IaaS supplies virtual machines and storage, so the guest operating system and patching stay with the customer; PaaS supplies a runtime and managed databases, so the customer owns only the code; SaaS supplies the application itself, such as cloud mail or shared documents.
The responsibility line is what exams test: after a leak, an unpatched system under IaaS is the customer’s failure, while the runtime under PaaS belongs to the provider.
School examples: a two-week exam registration site belongs on PaaS because traffic is short and concentrated, while long-term backups are cheaper on object storage or IaaS.
The responsibility line is what exams test: after a leak, an unpatched system under IaaS is the customer’s failure, while the runtime under PaaS belongs to the provider.
School examples: a two-week exam registration site belongs on PaaS because traffic is short and concentrated, while long-term backups are cheaper on object storage or IaaS.
Ask students: what the school still owns after choosing PaaS (application logic, accounts and permissions, the data itself).
Scene 3: Availability multiplies; it does not add
A single VM carries a 99.9% SLA, and a month is 43,200 minutes, so downtime is capped at 43.2 minutes; at 99.95% it is 21.6 minutes.
An application that needs both the VM and the database chains them, so availability is 0.999 times 0.995, which is 99.4005%, giving 259 minutes a month, about 4.3 hours, worse than either figure alone.
So add redundancy with two parallel VMs, or buy a stronger database SLA; a provider quoting 99.9% does not make your system 99.9% available.
An application that needs both the VM and the database chains them, so availability is 0.999 times 0.995, which is 99.4005%, giving 259 minutes a month, about 4.3 hours, worse than either figure alone.
So add redundancy with two parallel VMs, or buy a stronger database SLA; a provider quoting 99.9% does not make your system 99.9% available.
Ask students: why every extra layer lowers overall availability (all links must hold at once).
04 Question types and worked steps
| Type | Example | Steps | Answer |
|---|---|---|---|
| Calculation | Hosts needed | VMs divided by density, rounded up | 7 hosts |
| Calculation | On-premises monthly cost | Depreciation plus power, times hosts | HK$8,330.56 |
| Calculation | SLA downtime | 43,200 minutes times the failure share | 43.2 minutes |
| Essay | Own or rent | Cost, elasticity, control, compliance | Write a hybrid |
Sanity check (teach this): Round the host count up: 40 divided by 6 is 6.67, so buy 7, not 6.
Convert power first: 220 W times 24 hours times 30 days, divided by 1,000, gives kWh.
Chained availability multiplies; 99.9 plus 99.5 is not 199.4 but below either figure.
An SLA covers only the provider side, never your network, your code or human error.
Convert power first: 220 W times 24 hours times 30 days, divided by 1,000, gives kWh.
Chained availability multiplies; 99.9 plus 99.5 is not 199.4 but below either figure.
An SLA covers only the provider side, never your network, your code or human error.
05 Independent practice (3 questions)
| Question | Answer | What to watch |
|---|---|---|
| Hosts for 40 VMs at six per host | 7 hosts | Divide and round up |
| Monthly downtime at 99.9% | 43.2 minutes | 43,200 times 0.001 |
| Availability of 99.9% chained with 99.5% | 99.4005% | Multiply the two ratios |
06 Common mistakes and what to say
| Mistake | Student writes | Say this |
|---|---|---|
| Rounding the host count down | Answering 6 hosts | A VM cannot be halved, so the host count always rounds up. |
| Adding SLAs together | Answering 199.4% for two layers | Chained services multiply, so the total is weaker than the weakest link. |
| Blurring IaaS and PaaS duties | Answering that cloud removes all admin | Under IaaS the operating system and patches are still yours. |
| Comparing only the monthly bill | Answering that on-premises always wins | Add peaks, scaling time and four-year depreciation risk. |
07 Homework (paper, section and question numbers)
- Secondary ICT chapter index — All chapters and practice
- All lesson-prep guides — Compare other subjects
08 Exit check (last 5 minutes)
| Question | Answer | Decision |
|---|---|---|
| Hosts for 40 VMs | 7 hosts | All three right: move to network security |
| Downtime at 99.9% | 43.2 minutes | One wrong: reteach percentages |
| Chained availability | 99.4005% | Two wrong: return to act three |
09 Tutor FAQ
Must a school move everything to the cloud?
No. Steady load may be cheaper on site; decide on cost, elasticity and control.
Are virtualisation and cloud the same?
No. Virtualisation is a technique; cloud is a pay-as-you-go service model, and a private room can be virtualised too.
Is a higher SLA always better?
Check its scope and credits, and remember that chaining layers pulls the overall figure down.
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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.
