Our AWS RDS cost jumped unexpectedly over the last 2 days, even though usage and traffic were unchanged.
What went wrong?
After a deep dive, we found the root cause:
👉 Our Reserved DB Instance (RI) had expired.
Once the RI expired, the database automatically fell back to On-Demand pricing, which is significantly more expensive.
The fix
We immediately purchased a new Reserved DB Instance matching:
- PostgreSQL
- Instance class
- Multi-AZ
- Region
This was a billing-only change:
- No downtime
- No restarts
- No configuration changes
The result
💰 ~30–40% reduction in DB instance cost
📉 Monthly spend became predictable again
⚡ Savings applied instantly
Key takeaway
RI expirations are silent, but their cost impact is immediate.
Always monitor:
- RI expiration dates
- Cost Explorer anomalies
- Billing alerts
A missed RI renewal can cause instant cost spikes, even for stable workloads.
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