AWS Backup cost for EFS gone up after reducing backup intensity

The cost increase you're experiencing despite reducing backup frequency and retention period is likely due to how AWS Backup handles incremental backups for EFS. This is a common issue related to retention policy configuration.

When you reduced your retention period to just 3 days, you may have inadvertently created a situation where AWS Backup cannot efficiently create incremental backups. AWS recommends configuring your retention policy with a warm storage duration of at least one week to avoid additional charges.

Here's what's likely happening:

• With only 3 days retention, your backups might be removed from warm storage too quickly
• When the next backup runs, AWS Backup cannot create an incremental backup (which only stores changes) because the previous backup is no longer available in warm storage
• This forces AWS Backup to create full backups each time, significantly increasing your costs

To resolve this issue:

1. Increase your retention period to at least 7 days, even if you're only backing up 3 times per week
2. This allows AWS Backup to properly create incremental backups, which only store the changed data
3. The cost savings from incremental backups will likely outweigh the cost of slightly longer retention

Additionally, be aware that when you transition backups to cold storage, AWS Backup handles this differently for EFS. Only data that no longer exists in the most recent warm backup will transition to cold storage, while data that still exists remains in warm storage to support incremental backups.
Sources
AWS Backup - EFS Transition to Cold Storage - Incremental Backup | AWS re:Post
Metering, costs, and billing for AWS Backup - AWS Backup

AWS Backup costs for EFS are primarily driven by the total amount of data stored in the backup vault (storage costs) and the number of backup jobs performed (request costs), so while reducing frequency and retention seems like it should lower costs, the increase likely comes from either a data growth period coinciding with your schedule change or orphaned snapshots not being cleaned up - first, check your backup vault for old snapshots that weren't deleted due to retention rule issues, ensure you're not paying for "warm" or "cold" storage tiers unintentionally, and verify that your backup plan is actually deleting old backups (sometimes manual backups or previous schedule backups remain outside the automated lifecycle), then consider implementing a tag-based cleanup policy and monitor via AWS Backup Audit Manager to identify cost anomalies.