EBS snapshot cost: how to estimate storage from change rate
EBS snapshots are incremental. That means the “billable snapshot storage” is usually dominated by how much data changes over time and how long you retain snapshots, not just the raw size of the volume. A simple planning model is enough to avoid surprises.
Step 1: group volumes into classes
Avoid a single average. Use 3–5 classes based on workload and churn.
- Root volumes (low churn)
- App data volumes (moderate churn)
- Databases / write-heavy volumes (high churn)
Step 2: estimate daily change rate
Daily change rate is the percent of the volume that changes each day. If a 1,000 GB volume has a 2% change rate, a first-order estimate is ~20 GB of changed blocks per day.
- Low churn: ~0.5% to 1%/day
- Moderate churn: ~2% to 5%/day
- High churn: 5%+/day (logs, temp files, write-heavy DB workloads)
If you do not know churn, start with a range and validate later. The goal is sensitivity, not perfect accuracy on day one.
Step 3: model retention (what creates the steady state)
First-order steady-state model: snapshot stored GB ~= base + daily_delta_GB × (retention_days - 1)
- base: approximately the volume size (initial snapshot baseline)
- daily_delta_GB: volume_GB × change_rate
Worked example
- Volume size: 1,000 GB
- Change rate: 2%/day -> daily delta ~20 GB/day
- Retention: 30 days
- Stored snapshot GB ~ 1,000 + 20 × 29 ~ 1,580 GB (order-of-magnitude)
Real snapshot storage can differ by implementation and compression. Use this to budget, then validate against actual billed snapshot storage.
Copies and replication (easy to forget)
- Cross-region snapshot copies add another stored GB stream.
- Cross-account copies add storage in the destination account too.
- “Two copies” often means roughly two snapshot storage baselines.
Turn it into cost
Cost control levers (safe order)
- Retention: reduce retention for low-value snapshots first.
- Churn: move logs/temp/cache off the volume if feasible.
- Copies: keep the number of copies aligned to actual RPO/RTO requirements.
Validation checklist
- Measure snapshot stored GB by volume class for a real month and compare to the model.
- Validate which volumes have the highest churn (those dominate deltas).
- Validate retention and copy policies match RPO/RTO and compliance requirements.
- After changes, validate restore workflows (test restore, not just theory).
Sources
- EBS snapshots: docs.aws.amazon.com
- EBS pricing: aws.amazon.com/ebs/pricing