Why aren't my RDS reservations being used?

Question

My understanding is that RDS Reserved Instances should nearly immediately reduce daily costs, assuming the instance type, size and region match. However, I'm seeing the costs jump due to the reservations being procured but seemingly not matched.

When I view the reservation details, they are as follows:
**Reservation 1:**
Product description: RDS MariaDB
Quantity: 1
Remaining Days: 363
Multi-AZ deployment: No
Region: US East (Ohio)
Status: Active
DB Instance Class: db.m8g.2xlarge

**Reservation 2:**
Product description: RDS MariaDB
Quantity: 1
Remaining Days: 363
Multi-AZ deployment: Yes
Region: US East (Ohio)
Status: Active
DB Instance Class: db.m8g.2xlarge

The 2 RDS dbs I expected them to match are as follows: 
**DB1**
MariaDB
us-east-2c
db.m8g.2xlarge
Multi-AZ: Yes

**DB2**
MariaDB
us-east-2b
db.m8g.2xlarge
Multi-AZ: No

What am I missing or where did I go wrong?

re:Post Agent · Answer

There are a few potential reasons why your RDS Reserved Instances (RIs) might not be applying as expected:

1. **m8g instance type reservations availability**: Based on recent information, reserved instances for the m8g class in Amazon RDS may not yet be fully available for purchase. The m8g instances, which use AWS Graviton4 processors, were recently introduced for RDS for PostgreSQL, MySQL, and MariaDB. While these instances are available for on-demand usage, there is typically a delay between when new instance types are launched and when they become available for reserved instance purchases. If you've managed to purchase m8g reservations but they're not applying, this could be a system issue.

2. **Verification needed**: You should verify that your RIs are active by running the AWS CLI command `aws rds describe-reserved-db-instances` in your region. This will show if your reservations are properly active and configured.

3. **Automatic application**: RDS Reserved Instances are applied automatically to matching instances. If everything is configured correctly and they're still not applying, you may need to open a support ticket with AWS.

4. **Matching criteria**: Double-check that all attributes match exactly between your reservations and running instances. For RDS, this includes:
- Instance class (db.m8g.2xlarge)
- Region (US East Ohio/us-east-2)
- Database engine (MariaDB)
- Multi-AZ setting (Yes/No)

Based on the information you've provided, your reservations and instances appear to match correctly. Since you're seeing increased costs rather than savings, and your reservations show as "Active," this might be an issue that requires AWS Support intervention to resolve.  
**Sources**  
[RDS m8g reserved instances | AWS re:Post](https://repost.aws/questions/QUy0W9rALQQM6whFvqfXiWng/rds-m8g-reserved-instances)  
[How to apply RI (Reserved instance) in RDS? My RI is not applied | AWS re:Post](https://repost.aws/questions/QUYJiShHbGR3a5kL1n-Pn-5A/how-to-apply-ri-reserved-instance-in-rds-my-ri-is-not-applied)

Why aren't my RDS reservations being used?

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