EC2 Auto Scaling now fills your reserved capacity first!
Hey there, it's Shiichan! Today I found a nice little update to Amazon EC2 Auto Scaling, so let me walk you through it.
AWS What's New
aws.amazon.com
What was announced?
According to AWS's What's New, Amazon EC2 Auto Scaling added a new Availability Zone (AZ) distribution strategy called reservations-then-balanced. Just like the name says, it first launches instances into your capacity reservations, and then spreads the rest evenly across your Availability Zones. It has been available since June 30, 2026 in all AWS commercial Regions.
The story so far
Until now, Auto Scaling mostly spread instances evenly across multiple AZs. That's great for resilience, but if you had already paid up front for capacity — like On-Demand Capacity Reservations (ODCRs) or Capacity Blocks — Auto Scaling wouldn't necessarily prioritize using it. So you could end up with reserved capacity sitting only partly filled, which felt like a bit of a waste.
What changes
When you pick the new reservations-then-balanced strategy, Auto Scaling goes and fills your reserved capacity first. It covers not only ODCRs but also Capacity Blocks and Interruptible Capacity Reservations. Once the reservations are used up, the rest gets balanced across your AZs just like before, so you keep Auto Scaling's operational simplicity and resilience. It's a quietly handy update for anyone who wants to actually use up the capacity they paid for.
Dive Deep
You set it up by choosing the distribution strategy in the AvailabilityZoneDistribution configuration of your Auto Scaling group. You can target reservations by a Capacity Reservation Group ARN, or by listing individual Capacity Reservation IDs. There's no additional charge — you keep paying standard EC2 pricing for your reservations and any On-Demand or Spot instances. If you want the details, check out the Amazon EC2 Auto Scaling User Guide.
Wrap-up
- Amazon EC2 Auto Scaling got a new AZ distribution strategy called reservations-then-balanced
- It fills reserved capacity (ODCRs / Capacity Blocks / Interruptible Capacity Reservations) first, then balances the rest across your AZs
- You configure it via
AvailabilityZoneDistribution, at no extra charge, in all AWS commercial Regions - Perfect for infra folks who've been frustrated by leftover reserved capacity!
