Switching from RDS On-Demand to RDS Reserved Instance questions

Question

Hi re:Post,

We understand that Reserved Instances is a billing/cost construct and does not require us to migrate , move , or switch over anything regarding our (34) RDS PostgreSQL instances in order to take advantage of R.I. costs and savings.

We have been reading:

https://aws.amazon.com/rds/reserved-instances/

and

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithReservedDBInstances.html#USER_WorkingWithReservedDBInstances

Our (34) RDS instances are split up among (3) classes of On-Demand instance pricing:

(8)  db.m5.large 
	(23) db.m6i.large
	(3)  db.m6i.xlarge
               All are Multi-AZ deployed (1 standby per instance)
               All are US East (Ohio)

We are considering no-upfront, 1 year commitment for starters.

q1) When we start the purchase process, do we go through the process 1x for each of our classes of RDS instances?  For example, our "db.m5.large" instance class we would specify (8) for the "Number of DB Instances" for that class, purchase all (8) and finish.  Then we would start the process over again for our (23) db.m6i.large and then again for our (3) db.m6i.xlarge, correct?

q2) Say for one of our instances in the "db.m5.large" class or any class, we end up needing it for 1 year and 6 months.  How does that affect costs?  What if it's only 6-9 months?

q3) I went through the initial first page of "Purchase reserved DB instance" to get idea of the inputs and process.  What are the next steps after I press "Next" at the bottom of that page? (2nd screenshot).

![Enter image description here](/media/postImages/original/IMzINazNnOTBWpp9Lr_9Ev2A)

![Enter image description here](/media/postImages/original/IMrz8YKL1CRnG29b3VDVriMA)

Thank you for your time and help!

Best Regards,

Donald

Kallu · Accepted Answer

Q2: Your break-even point with reservations is somewhere around 7-9 months depending what instance type and purchase option your are using -> If you're confident that usage will be atleast 9 (out of 12) months it would be cheaper to buy RI than use on-demand. And this doesn't have to be 9 months in a row but can split anyway you like as long as total is somewhere around 75% of year.

Calculators at https://www.concurrencylabs.com/blog/guide-to-rds-reserved/ might be helpful to compare your options.

re:Post Agent · Answer

Thank you for your detailed questions about switching from RDS On-Demand to RDS Reserved Instances. I'll address each of your questions in order.

q1) Yes, you are correct. You would need to go through the purchase process separately for each instance class. For your db.m5.large instances, you would specify 8 for the "Number of DB Instances" and complete that purchase. Then you would repeat the process for your 23 db.m6i.large instances, and again for your 3 db.m6i.xlarge instances. Each purchase would be specific to the instance class, deployment type (Multi-AZ in your case), and region (US East Ohio).

q2) Reserved Instances are purchased for a fixed term of either one or three years. If you need an instance for 1 year and 6 months, you would benefit from the Reserved Instance pricing for the first year, and then pay On-Demand rates for the additional 6 months. If you only need the instance for 6-9 months, you would still be committed to paying for the full year of the Reserved Instance. In this case, it might be more cost-effective to use On-Demand instances if you're certain you won't need the instance for the full year. It's important to carefully consider your long-term needs when purchasing Reserved Instances.

q3) After pressing "Next" on the initial page, you would typically see a summary of your selection and the associated costs. This would include details such as the total upfront cost (if any), the monthly recurring charges, and the effective hourly rate. You would have the opportunity to review these details and confirm your purchase. The exact steps may vary slightly, but generally, you would review the order summary, agree to the terms, and then finalize the purchase.

Remember that while Reserved Instances offer significant savings for steady, long-term use, they also represent a commitment. Make sure you're confident in your long-term needs before making the purchase. Also, keep in mind that Reserved Instances are a billing construct and don't require any migration or changes to your existing instances - the discount is automatically applied to matching running instances in your account.  
**Sources**  
[Amazon RDS Reserved Instances | Cloud Relational Database | Amazon Web Services](https://aws.amazon.com/rds/reserved-instances/)  
[On-Demand DB instances for Amazon RDS - Amazon Relational Database Service](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_OnDemandDBInstances.html)  
[Purchasing reserved DB instances for Amazon RDS - Amazon Relational Database Service](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithReservedDBInstances.WorkingWith.html)  
[Reserved Instances](https://aws.amazon.com/aws-cost-management/aws-cost-optimization/reserved-instances/)

Switching from RDS On-Demand to RDS Reserved Instance questions

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