Reserved Instances: Complete Guide for AWS SysOps Administrator Associate Exam
Why Reserved Instances Are Important
Reserved Instances (RIs) represent one of the most significant cost-saving mechanisms in AWS, offering up to 72% discount compared to On-Demand pricing. For the SysOps Administrator Associate exam, understanding RIs is crucial because cost optimization is a core domain, and questions frequently test your knowledge of when and how to apply different purchasing options.
What Are Reserved Instances?
Reserved Instances are a billing discount applied to the use of On-Demand Instances in your account. They are not physical instances but rather a commitment to use a specific instance configuration for a 1-year or 3-year term in exchange for reduced hourly rates.
There are three types of Reserved Instances:
Standard Reserved Instances: Offer the highest discount (up to 72%) but provide limited flexibility. You can modify Availability Zone, scope, networking type, and instance size within the same instance family.
Convertible Reserved Instances: Offer lower discounts (up to 54%) but allow you to change instance family, operating system, tenancy, and payment options during the term.
Scheduled Reserved Instances: Allow you to reserve capacity for specific time windows (daily, weekly, or monthly) - ideal for predictable recurring workloads.
How Reserved Instances Work
Payment Options:
- All Upfront: Pay the entire cost upfront for maximum discount
- Partial Upfront: Pay a portion upfront with reduced hourly rate
- No Upfront: No upfront payment, smallest discount but still significant savings
Scope Options:
- Regional: Applies to any Availability Zone in the region, provides capacity reservation flexibility
- Zonal: Applies to a specific Availability Zone, provides capacity reservation guarantee
Instance Size Flexibility:
Regional RIs with Linux/Unix and default tenancy automatically apply discounts to any instance size within the same instance family using a normalization factor.
Key Concepts for the Exam
1. Capacity Reservation: Only Zonal RIs provide guaranteed capacity. Regional RIs do not guarantee capacity but offer size flexibility.
2. RI Marketplace: You can sell unused Standard RIs on the Reserved Instance Marketplace if your needs change.
3. Billing: RIs are applied automatically to matching running instances. The discount applies per hour regardless of whether instances are running.
4. Normalization Factor: Instance sizes are normalized (nano=0.25, micro=0.5, small=1, medium=2, large=4, xlarge=8, etc.) to allow flexible application of RI discounts.
Exam Tips: Answering Questions on Reserved Instances
Tip 1: When a question mentions steady-state, predictable workloads running 24/7, Reserved Instances are typically the correct answer for cost optimization.
Tip 2: If the scenario requires flexibility to change instance types during the commitment period, choose Convertible RIs over Standard RIs.
Tip 3: For questions about guaranteed capacity in a specific Availability Zone, remember that only Zonal RIs provide this guarantee.
Tip 4: When asked about maximum savings, the answer involves: 3-year term + All Upfront payment + Standard RI.
Tip 5: If a question discusses selling unused reservations, remember that only Standard RIs can be sold on the RI Marketplace, not Convertible RIs.
Tip 6: For workloads that run only during specific predictable time windows, Scheduled Reserved Instances are the appropriate choice.
Tip 7: Questions comparing Savings Plans vs RIs: Savings Plans offer more flexibility across instance families and regions, while RIs are more specific but can provide capacity reservations.
Tip 8: Remember that RIs apply at the account level by default, but in AWS Organizations, RI sharing can be enabled or disabled across member accounts.