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Spot VMs for cost savings in Azure - Time & Space Complexity

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Time Complexity: Spot VMs for cost savings
O(n)
Understanding Time Complexity

We want to understand how the time to deploy and manage Spot VMs changes as we increase the number of VMs.

How does adding more Spot VMs affect the number of operations and time needed?

Scenario Under Consideration

Analyze the time complexity of creating multiple Spot VMs in Azure.


// Create multiple Spot VMs
for (int i = 0; i < vmCount; i++) {
    var vm = new VirtualMachine()
        .WithSpotPriority(true)
        .WithEvictionPolicy(VirtualMachineEvictionPolicy.Deallocate)
        .Create();
}

This sequence creates a number of Spot VMs, each with a spot priority and eviction policy set.

Identify Repeating Operations

Look at what repeats as we add more VMs.

  • Primary operation: Creating a single Spot VM with specific settings.
  • How many times: Once per VM, so vmCount times.
How Execution Grows With Input

Each VM creation is a separate operation, so more VMs mean more operations.

Input Size (vmCount)Approx. API Calls/Operations
1010 VM creation calls
100100 VM creation calls
10001000 VM creation calls

Pattern observation: The number of operations grows directly with the number of VMs.

Final Time Complexity

Time Complexity: O(n)

This means the time to create Spot VMs grows linearly as you add more VMs.

Common Mistake

[X] Wrong: "Creating multiple Spot VMs happens all at once, so time stays the same no matter how many VMs."

[OK] Correct: Each VM creation is a separate call and takes time, so more VMs mean more total time.

Interview Connect

Understanding how operations scale with input size helps you design efficient cloud deployments and explain your reasoning clearly.

Self-Check

What if we used a batch API to create multiple Spot VMs at once? How would the time complexity change?

Practice

(1/5)
1. What is the main benefit of using Azure Spot VMs?
easy
A. They automatically scale without any configuration.
B. They guarantee 100% uptime for critical applications.
C. They provide cheaper compute by using spare capacity.
D. They offer unlimited storage space for virtual machines.

Solution

  1. Step 1: Understand Spot VM purpose

    Spot VMs use unused cloud capacity to offer lower prices.

  2. Step 2: Compare benefits

    Unlike regular VMs, Spot VMs are cheaper but can be evicted when capacity is needed.

  3. Final Answer:

    They provide cheaper compute by using spare capacity. -> Option C

  4. Quick Check:

    Spot VMs = cheaper compute [OK]
Hint: Spot VMs save cost by using spare capacity [OK]
Common Mistakes:
  • Thinking Spot VMs guarantee uptime
  • Confusing Spot VMs with auto-scaling
  • Assuming Spot VMs provide extra storage
2. Which of the following is the correct way to set a Spot VM priority in Azure CLI?
easy
A. az vm create --priority High
B. az vm create --priority Spot
C. az vm create --spot-priority true
D. az vm create --enable-spot

Solution

  1. Step 1: Recall Azure CLI syntax for Spot VMs

    The correct parameter to set Spot VM priority is --priority Spot.

  2. Step 2: Evaluate options

    The other options use incorrect or non-existent flags.

  3. Final Answer:

    az vm create --priority Spot -> Option B

  4. Quick Check:

    Spot VM priority flag = --priority Spot [OK]
Hint: Use --priority Spot to create Spot VMs [OK]
Common Mistakes:
  • Using incorrect flags like --spot-priority
  • Setting priority to High instead of Spot
  • Assuming --enable-spot is valid
3. Consider this Azure CLI command to create a Spot VM: 
az vm create --name mySpotVM --image UbuntuLTS --priority Spot --max-price 0.05
What happens if the current Spot price exceeds 0.05 USD/hour?
medium
A. The VM is converted to a regular VM automatically.
B. The VM continues running at the higher price.
C. The VM price is capped at 0.05 USD/hour but runs normally.
D. The VM is evicted (stopped) automatically.

Solution

  1. Step 1: Understand max-price setting

    The max-price limits the Spot VM cost; if price rises above it, eviction occurs.

  2. Step 2: Analyze behavior when price exceeds max-price

    Spot VMs are stopped or deallocated automatically when price exceeds max-price.

  3. Final Answer:

    The VM is evicted (stopped) automatically. -> Option D

  4. Quick Check:

    Price > max-price = VM eviction [OK]
Hint: Spot VM stops if price goes above max-price [OK]
Common Mistakes:
  • Thinking VM keeps running at higher price
  • Assuming price is capped automatically
  • Believing VM converts to regular VM
4. You tried to create a Spot VM with this command: 
az vm create --name testVM --image UbuntuLTS --priority Spot --max-price -2
What is the issue with this command?
medium
A. max-price cannot be negative; it causes an error.
B. Priority Spot is invalid syntax.
C. Image UbuntuLTS is not supported for Spot VMs.
D. VM name testVM is reserved and cannot be used.

Solution

  1. Step 1: Check max-price parameter rules

    max-price must be greater than or equal to -1; other negative values are invalid.

  2. Step 2: Identify error cause

    Using -2 for max-price causes a validation error during VM creation.

  3. Final Answer:

    max-price cannot be negative; it causes an error. -> Option A

  4. Quick Check:

    max-price < -1 = error [OK]
Hint: max-price must be >= -1 [OK]
Common Mistakes:
  • Using negative max-price values
  • Thinking Spot priority syntax is wrong
  • Assuming UbuntuLTS is unsupported
5. You want to run a batch job that can pause and resume without losing progress. Which Spot VM configuration helps minimize cost while ensuring the job restarts automatically if evicted?
hard
A. Set VM priority to Spot, max-price to a low value, and enable automatic redeployment.
B. Use regular VMs with high priority and no max-price limit.
C. Set VM priority to Spot with max-price set to -1 to avoid eviction.
D. Use Spot VMs without max-price and disable automatic redeployment.

Solution

  1. Step 1: Choose Spot VM with cost control

    Setting priority to Spot and max-price low saves cost but risks eviction.

  2. Step 2: Enable automatic redeployment

    Automatic redeployment restarts the VM if evicted, ensuring job resumes.

  3. Final Answer:

    Set VM priority to Spot, max-price to a low value, and enable automatic redeployment. -> Option A

  4. Quick Check:

    Spot + max-price + auto redeploy = cost saving + restart [OK]
Hint: Spot + low max-price + auto redeploy = cost + restart [OK]
Common Mistakes:
  • Using negative max-price to avoid eviction
  • Disabling automatic redeployment
  • Choosing regular VMs for cost savings