Can Two Single-AZ t2.medium Reserved Instances Cover Costs of a Multi-AZ RDS t2.medium Instance Deployment?

Question

Hello,

I am currently utilizing an Amazon RDS t2.medium instance in a Multi-AZ instance deployment configuration. It's important to note that I am referring to a Multi-AZ instance deployment, not a Multi-AZ cluster deployment. Multi-AZ instance deployment essentially has two nodes - a primary node and a standby replica.

I already have one Single-AZ t2.medium Reserved Instance which I believe is covering the cost of the primary node. I am considering purchasing an additional Single-AZ t2.medium Reserved Instance to offset the cost of the standby replica in the Multi-AZ instance deployment.

Can you please confirm if purchasing two Single-AZ t2.medium Reserved Instances will effectively cover both nodes of a t2.medium Multi-AZ RDS instance deployment, thereby providing cost savings?

Thank you in advance for your assistance.

Best regards,

Accepted Answer

Refer **How billing works** and **Reserved Instance Size Flexibility** at [Amazon RDS Reserved Instances](https://aws.amazon.com/rds/reserved-instances/).

Amazon RDS Reserved Instances provide size flexibility for the MySQL, MariaDB, PostgreSQL, and Amazon Aurora database engines as well as the “Bring your own license” (BYOL) edition of the Oracle database engine. With size flexibility, your RI’s discounted rate will automatically apply to usage of any size in the instance family (using the same database engine). Size flexibility does not apply to Microsoft SQL Server and the License Included (LI) edition of Oracle.

For example, let’s say you purchased a db.m4.2xlarge MySQL RI in US East (N. Virginia). The discounted rate of this RI can automatically apply to 2 db.m4.xlarge MySQL instances without you needing to do anything.

The RI discounted rate will also apply to usage to both Single-AZ and Multi-AZ configurations for the same database engine and instance family. For example, let’s say you purchased a db.r3.large PostgreSQL Single-AZ RI in EU (Frankfurt). The discounted rate of this RI can automatically apply to 50% of the usage of a db.r3.large PostgreSQL Multi-AZ instance in the same region.

Your second AZ instance would be covered as it would still be in same region.

Answer

I think it would apply if you purchase another single AZ reserved instance.  
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithReservedDBInstances.html  
> Reserved DB instance benefits also apply for both Multi-AZ and Single-AZ configurations. Flexibility means that you can move freely between configurations within the same DB instance class type. For example, you can move from a Single-AZ deployment running on one large DB instance (four normalized units per hour) to a Multi-AZ deployment running on two small DB instances (2*2 = 4 normalized units per hour).

You may want to open a case with AWS support with "Account and billing" to be sure.

Can Two Single-AZ t2.medium Reserved Instances Cover Costs of a Multi-AZ RDS t2.medium Instance Deployment?

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