Amazon EC2 Reserved Instances and Other AWS Service Reservation Models
AWS Whitepaper

How Discounts Are Applied

The application of Amazon EC2 Reserved Instances is based on instance attributes, including the following:

  • Instance type – Instance types comprise varying combinations of CPU, memory, storage, and networking capacity (e.g., m4.xlarge). This gives you the flexibility to choose the appropriate mix of resources for your applications, such as compute optimized, storage optimized, and so on. Each instance type includes one or more instance sizes, enabling you to scale your resources to the requirements of your target workload.

  • Platform – You can purchase Reserved Instances for Amazon EC2 instances running Linux or UNIX, SUSE Linux, Red Hat Enterprise Linux, Windows Server, and Microsoft SQL Server platforms.

  • Tenancy – Reserved Instances can be default tenancy or dedicated tenancy.

  • Regional or Zonal – See Maximizing Utilization with Size Flexibility in Regional Reserved Instances.

If you purchase a Reserved Instance and you already have a running instance that matches the attributes of the Reserved Instance, the billing benefit is immediately applied. You don’t have to restart your instances. If you do not have an eligible running instance, launch an instance and ensure that you match the same criteria that you specified for your Reserved Instance. For more information, see Using Your Reserved Instances.