Where at-rest encryption applies (on-disk data):

• Data stored on SSDs (solid-state drives) in data tiering enabled clusters is always encrypted by default.
• Disk during sync, backup and swap operations
• Backups stored in S3

See also

• Data tiering - https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/data-tiering.html
• Why does swapping occur in ElastiCache? https://aws.amazon.com/premiumsupport/knowledge-center/elasticache-swap-activity/

• Let’s Architect! Leveraging in-memory databases, AWS, 2023-09-13