Azure Reserved Instances - Deep Dive

Overview - Azure Reserved Instances

What is it?

Azure Reserved Instances are a way to save money by committing to use certain virtual machines or other resources in Microsoft Azure for a fixed period, usually one or three years. Instead of paying for resources by the hour, you pay upfront or commit to a lower rate, which reduces costs. This helps businesses plan their cloud spending and get discounts compared to pay-as-you-go pricing.

Why it matters

Without Azure Reserved Instances, businesses pay higher prices for cloud resources because they pay only for what they use each hour. This can lead to unpredictable costs and higher bills. Reserved Instances solve this by offering a cheaper, predictable price for resources you know you will use long-term, helping companies save money and budget better.

Where it fits

Before learning about Azure Reserved Instances, you should understand basic cloud concepts like virtual machines and pay-as-you-go pricing. After this, you can explore other cost management tools in Azure like Azure Hybrid Benefit or Azure Spot Instances to optimize spending further.

Mental Model

Core Idea

Azure Reserved Instances let you pre-pay or commit to using cloud resources for a long time to get a lower price than paying hourly.

Think of it like...

It's like buying a yearly gym membership instead of paying for each visit; you pay upfront and get a discount because you commit to using the gym regularly.

┌─────────────────────────────┐
│       Azure Reserved        │
│          Instances          │
├─────────────┬───────────────┤
│ Commit Time │ 1 or 3 years  │
├─────────────┼───────────────┤
│ Payment     │ Upfront or    │
│             │ monthly       │
├─────────────┼───────────────┤
│ Benefit     │ Lower price   │
│             │ than pay-as-  │
│             │ you-go        │
└─────────────┴───────────────┘

Build-Up - 7 Steps

1

Foundation

Understanding Pay-As-You-Go Pricing

Concept: Learn how Azure charges for resources when you pay by the hour without any commitment.

Azure charges for virtual machines and other resources based on how long you use them. This is called pay-as-you-go pricing. You start a VM, and the clock runs until you stop it. You pay for every hour or minute it runs, which can add up if you use resources a lot.

Result

You get flexible usage but may pay more if you run resources continuously.

Understanding pay-as-you-go pricing helps you see why committing to reserved instances can save money.

2

Foundation

What Are Azure Reserved Instances?

Concept: Introduce the idea of committing to use resources for a set time to get discounts.

Azure Reserved Instances let you reserve virtual machines or other resources for one or three years. You pay upfront or commit to a lower monthly rate. Because you promise to use the resource for a long time, Azure gives you a discount compared to pay-as-you-go prices.

Result

You pay less overall if you use the resource continuously during the reserved period.

Knowing that reserved instances trade flexibility for cost savings is key to using them wisely.

3

Intermediate

Types of Azure Reserved Instances

Concept: Explore different resource types and payment options available for reserved instances.

Azure Reserved Instances apply mainly to virtual machines but also to other services like SQL Database compute capacity. You can choose to pay all upfront, partially upfront, or monthly. The longer the commitment, the bigger the discount. You also select the VM size, region, and operating system when reserving.

Result

You understand how to pick the right reserved instance for your needs and budget.

Knowing payment and resource options helps tailor reserved instances to your workload and cash flow.

4

Intermediate

How Azure Reserved Instances Apply Discounts

🤔Before reading on: Do you think reserved instances create new resources or just discount existing usage? Commit to your answer.

Concept: Learn that reserved instances do not create resources but discount usage of matching resources.

Azure Reserved Instances do not create or allocate resources themselves. Instead, when you run a VM that matches your reserved instance (same size, region, OS), Azure automatically applies the discount to that usage. If you don't use the reserved instance, you lose the discount but still pay the reservation fee.

Result

You see that reserved instances are a billing discount, not extra capacity.

Understanding that reserved instances discount matching usage prevents confusion about resource availability and billing.

5

Intermediate

Flexibility and Exchange Options

🤔Before reading on: Can you change the VM size or region of a reserved instance after purchase? Commit to yes or no.

Concept: Explain how Azure allows some flexibility to exchange or cancel reserved instances.

Azure lets you exchange reserved instances if your needs change. For example, you can switch to a different VM size or region within the same VM family. You can also cancel reserved instances early but may pay a fee. This flexibility helps adapt to changing workloads while still saving money.

Result

You know how to adjust reserved instances to avoid wasting money on unused reservations.

Knowing exchange and cancellation options helps manage risk when committing to reserved instances.

6

Advanced

Combining Reserved Instances with Other Savings

🤔Before reading on: Do you think reserved instances discounts stack with Azure Hybrid Benefit? Commit to yes or no.

Concept: Learn how reserved instances work with other Azure cost-saving programs.

Azure Reserved Instances can be combined with Azure Hybrid Benefit, which lets you use existing Windows Server or SQL Server licenses to save more. The discounts stack, giving you bigger savings. However, reserved instances do not combine with Spot Instances or other temporary pricing models.

Result

You understand how to maximize savings by layering discounts.

Knowing how discounts combine helps optimize cloud costs strategically.

7

Expert

Billing and Usage Matching Internals

🤔Before reading on: Do you think reserved instance discounts apply only to the exact VM instance you reserved? Commit to yes or no.

Concept: Reveal how Azure matches usage to reserved instances dynamically across subscriptions and resource groups.

Azure uses a billing engine that matches your running VMs to reserved instances based on attributes like VM size, region, and OS. This matching happens automatically and can apply across subscriptions in the same billing account. This means you don't have to assign reserved instances to specific VMs manually. The system optimizes discount application to maximize your savings.

Result

You see that reserved instance discounts are flexible and managed by Azure's billing system, not tied to specific VMs.

Understanding the dynamic matching mechanism explains why reserved instances save money even if you change or move VMs.

Under the Hood

Azure Reserved Instances work by the billing system recognizing when you run a resource that matches a reserved instance you purchased. The system then applies a discounted rate to that usage instead of the standard pay-as-you-go rate. This matching considers VM size, region, OS, and subscription scope. The reservation itself is a billing construct, not a physical resource allocation.

Why designed this way?

This design allows Azure to offer cost savings without limiting customer flexibility. Instead of locking resources, Azure provides discounts on usage, letting customers change or stop VMs freely. It balances cost savings with operational freedom, avoiding wasted capacity and complex resource management.

┌───────────────┐       ┌─────────────────────┐       ┌───────────────┐
│ Reserved      │       │ Billing Engine      │       │ Running VM    │
│ Instance      │──────▶│ Matches Usage to RI │◀──────│ Usage Details │
│ (Commitment)  │       │ Applies Discount    │       │ (Size, Region)│
└───────────────┘       └─────────────────────┘       └───────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Do reserved instances guarantee you get a VM even if Azure is busy? Commit to yes or no.

Common Belief: Reserved instances reserve actual VM capacity, so you always get the VM when needed.

Reality: Reserved instances only provide a billing discount; they do not reserve physical capacity or guarantee availability.

Why it matters: Believing this can lead to unexpected downtime or capacity issues if you assume reserved instances guarantee resource availability.

Quick: Can you use reserved instances discounts on any VM size or region? Commit to yes or no.

Common Belief: Reserved instance discounts apply to any VM regardless of size or location.

Reality: Discounts only apply to VMs that match the reserved instance's size, region, and OS.

Why it matters: Misunderstanding this causes wasted money if you run VMs that don't match your reservations and miss out on discounts.

Quick: Do reserved instances automatically reduce your monthly bill even if you don't run any VMs? Commit to yes or no.

Common Belief: If you buy reserved instances, your bill goes down regardless of usage.

Reality: You pay for reserved instances whether you use matching VMs or not; discounts apply only when matching usage occurs.

Why it matters: Thinking otherwise can cause surprise charges for unused reservations.

Quick: Can you assign a reserved instance to a specific VM manually? Commit to yes or no.

Common Belief: You must assign reserved instances to specific VMs to get discounts.

Reality: Azure automatically applies reserved instance discounts to matching VMs across subscriptions; no manual assignment is needed.

Why it matters: Believing manual assignment is required can complicate management unnecessarily.

Expert Zone

1

Reserved instances discounts apply at the billing account level, allowing cross-subscription usage matching within the same billing scope.

2

The reservation term can be split or combined using exchanges, but cancellation incurs a fee, so planning is crucial.

3

Reserved instances do not cover all VM features; for example, ephemeral OS disks or certain VM series may have different reservation rules.

When NOT to use

Avoid reserved instances if your workloads are highly variable or short-lived; instead, use pay-as-you-go or Spot Instances for maximum flexibility and cost efficiency.

Production Patterns

Enterprises often combine reserved instances with Azure Hybrid Benefit and auto-scaling to optimize costs while maintaining performance and flexibility in production environments.

Connections

Subscription Billing Models

Builds-on

Understanding how Azure billing works at the subscription and billing account level helps grasp how reserved instances discounts are applied automatically.

Cloud Cost Optimization

Same pattern

Azure Reserved Instances are a key tool in cost optimization strategies, similar to reserved capacity in other clouds, showing a common approach to balancing cost and commitment.

Gym Memberships

Analogy

The concept of paying upfront for a service to get a discount is common in many fields, showing how commitment can reduce costs across domains.

Common Pitfalls

#1 Buying reserved instances without matching your actual VM usage.

Wrong approach:

Purchase reserved instance for VM size A in region X but run mostly VM size B in region Y.

Correct approach:

Analyze your VM usage patterns and buy reserved instances that match the VM sizes and regions you use most.

Root cause:

Misunderstanding that reserved instance discounts only apply to matching VM attributes.

#2 Assuming reserved instances guarantee resource availability.

Wrong approach:

Stop paying for on-demand VMs expecting reserved instances to provide capacity during spikes.

Correct approach:

Use reserved instances for cost savings but maintain capacity planning and scaling strategies for availability.

Root cause:

Confusing billing discounts with physical resource reservation.

#3 Not monitoring reserved instance usage and losing discounts.

Wrong approach:

Buy reserved instances and forget to check if your VMs match them over time.

Correct approach:

Regularly review reserved instance utilization reports and adjust reservations via exchanges or cancellations as needed.

Root cause:

Lack of ongoing cost management and monitoring.

Key Takeaways

Azure Reserved Instances offer significant cost savings by committing to use resources for one or three years.

They provide billing discounts, not physical resource reservations, so availability is not guaranteed.

Discounts apply only when running resources match the reserved instance's size, region, and OS.

Azure automatically applies reserved instance discounts across subscriptions within the same billing account.

Proper planning, monitoring, and flexibility options like exchanges help maximize savings and avoid wasted costs.