Rightsizing
A Reserved Instance is a commitment you cannot take back.
Buy a reservation for an instance that turns out to be idle, and the term still runs its course. Here's what a commitment locks in, per service, and the one caution Reserver attaches before you buy.
First, the commitment
A reservation on the wrong instance is the most expensive mistake in this workflow.
A Reserved Instance is a bet that an instance keeps running the way it runs today. Get that wrong, and you are not out a few dollars. You are locked into a rate for the full term. On most of the services Reserver covers, you cannot end that term early.
Reserver's own recommendations look at what a fleet is running, not just what it costs on demand. When every instance behind a recommendation has spent its recent history near idle, Reserver says so before you buy.
What a commitment cannot undo
Four reservation types. Four different amounts of room to change your mind.
None of the four gives you a plain cancel-and-refund. Two give you a narrow path to change course. Two give you none at all.
EC2 Standard Reserved Instance
You can modify the Availability Zone, the scope, or the instance size within the same family. You can also list it on the Reserved Instance Marketplace. That path carries limits: at least a month left in the term, 30 days active first, and a 12% fee. You cannot cancel it for a refund.
EC2 Convertible Reserved Instance
You can exchange it for a different family, operating system, or tenancy. The new one must be worth the same or more. It cannot be modified in place, and it cannot be listed on the Reserved Instance Marketplace at all.
RDS reserved DB instance
You cannot cancel it. Region, engine, and instance class are fixed at purchase. Delete the database instance it covers, and the upfront cost is still charged.
ElastiCache reserved node
Region, node type, and term are fixed at purchase and cannot change afterward. You cannot cancel it.
AWS's own modification guide covers the Standard RI path. The Convertible exchange guide covers the other EC2 path. It states plainly that a Standard RI cannot be exchanged, only modified. The Reserved Instance Marketplace guide states that RDS and ElastiCache reservations cannot be listed for sale at all. A Convertible RI cannot be listed either. RDS's own reserved DB instance guide states you cannot cancel one. You are billed the upfront cost whether you use it or not. ElastiCache's reserved node guide says the same. No cancellation, and Region, node type, and term are fixed for the life of the reservation.
What "idle" means here
Peak CPU over the window, not average CPU over the window.
An instance that averages low but spikes high is not idle. It is doing real work in short bursts, and a reservation on it still pays off. Reserver checks the peak so a bursty workload is never mistaken for one that is not needed at all.
The check runs over a trailing 14-day window of CloudWatch history, sampled once a day. An instance clears the check the moment even one day in that window shows a real peak. Only a fleet that stayed quiet the entire window gets the caution.
The rule: no signal is not idle
Missing utilization data is never read as low usage.
Reserver samples CPU, network in and out, database connections, and freeable memory, per instance, over the trailing window. A cache engine's own CPU metric stands in for plain CPU where it applies. The caution uses only the CPU signal, and only when it exists.
A recommendation carries the caution only under two conditions. Every contributing instance must have a CPU reading for the window. Every one of those readings must peak below 20%. A connection that has not finished its first scan might have no reading at all. Reserver treats that instance as unknown, not as idle, and the caution does not fire. An absence of evidence is not evidence of an idle fleet.
What it costs
The threshold, the window, and one year of a reservation nobody needed.
The same illustrative reserved rate the savings playbook prices a fleet with, run for a full term. No on-demand saving offsets it.
A reservation on covered, active capacity earns back what it costs. A reservation on capacity that turns out to be idle for the whole term earns back nothing. The rate is the same either way. Only the outcome differs.
How this scales with you
Checking CPU by hand works for a handful of instances, once.
A handful of instances is easy to check by eye before buying. The term is long, and the fleet is small, so one bad commitment still stands out on the bill. A caution on the recommendation is worth a second look.
Dozens of reservations across a few accounts move past what anyone checks by hand. A caution attached to the recommendation itself catches a low-CPU group the day it's proposed, not after the term has started.
Thousands of reservations across many accounts make a manual CPU check impossible before every purchase. A caution shown on the recommendation itself is the only way this scales past a handful of accounts.
Where Reserver fits
The caution rides on the recommendation, not a separate report.
Every recommendation Reserver proposes carries this check. It sits on the same page as the on-demand rate, the reserved rate, and the break-even date. See how the savings math is built, or see the product tour for the full recommendation flow.
What does a rightsizing caution mean?
Reserver attaches the note "consider rightsizing before reserving" to a recommendation. It fires when every instance behind that recommendation peaked below 20% CPU over the trailing 14 days. It is a caution, not a block. You still decide whether to buy.
Does Reserver resize the instance for me?
No. Reserver flags a recommendation as a candidate for rightsizing. It does not resize, stop, or change any instance. Any change to the instance itself is yours to make, on your own schedule.
What if some instances in the group have no utilization data?
No caution. Reserver only attaches the note when every contributing instance has a CPU signal, and all of those signals are low. Missing data on even one instance counts as unknown, never as idle.
Can I still buy a recommendation that carries this caution?
Yes. The gap and the offer grid render exactly as they would without it. The caution is a note next to the numbers, not a removed option.
Why does peak CPU matter more than average CPU here?
A bursty instance can average low and still peak high enough to justify its current size. Reserver checks the peak, so a short, real spike is never mistaken for idle.
Can I undo a Reserved Instance purchase if it turns out to be wrong?
It depends on the service, and never fully. An EC2 Standard RI can be modified or resold, with limits. An EC2 Convertible RI can only be exchanged. An RDS or ElastiCache reservation cannot be cancelled, modified, or resold at all.
Sources, fetched and confirmed on 2026-09-12: modifying EC2 Reserved Instances; exchanging Convertible Reserved Instances; the Reserved Instance Marketplace; working with RDS reserved DB instances; and ElastiCache reserved nodes.
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