Google PCA Practice Question: Analysing and Optimising Technical and Business Processes
A company wants to reduce costs for a batch analytics job that runs nightly for 4 hours on Compute Engine VMs. The job is fault-tolerant and can handle instance restarts. Which Compute Engine VM pricing model is MOST cost-effective?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Preemptible VMs
Preemptible VMs offer the lowest cost (up to 80% discount) and are ideal for fault-tolerant, short-lived batch workloads that can handle interruptions. Sustained use discounts apply automatically but require running a VM for at least 25% of a month. Committed use discounts require a 1- or 3-year commitment and are not as flexible for a short nightly job.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
3-year committed use discount (CUD)
Why it's wrong here
A 3-year CUD requires a continuous commitment billed for every hour of the term, so a nightly four-hour job pays for twenty unused hours daily. It is tempting because CUDs offer the deepest per-hour discount, and they suit workloads running continuously for the full commitment period.
- ✗
1-year committed use discount (CUD)
Why it's wrong here
A 1-year CUD still bills hourly for the whole commitment term, so a nightly four-hour job pays for twenty idle hours each day. It is tempting because the discount is substantial and the term is shorter than three years, but CUDs only pay off for workloads running continuously.
- ✗
Sustained use discounts
Why it's wrong here
Sustained use discounts apply automatically to instances running much of the billing month; a nightly four-hour job never accumulates enough monthly runtime to earn them. It is tempting because they require no commitment, but Spot VMs, which tolerate restarts, deliver far larger savings for fault-tolerant batch work.
- ✓
Preemptible VMs
Why this is correct
Preemptible VMs suit this fault-tolerant nightly batch job because they cost up to 80% less than standard instances, and the workload already tolerates restarts. The 24-hour maximum lifetime exceeds the 4-hour runtime, so forced preemption risk is acceptable, satisfying the cost-reduction constraint without disrupting analytics.
Go deeper
Related to this question
Learn chapter
Billing, Budgets, and Cost Management
Key term
Compute Engine
Compute Engine is Google Cloud's Infrastructure-as-a-Service (IaaS) offering that lets you create and run virtual machines on Google's infrastructure.
Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
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