what does AWS Storage Class Analysis cumulativeAccessRatio calculate?

The CumulativeAccessRatio in Amazon S3 Storage Class Analysis is a metric that represents the usage/byte heat on a given age group of objects. This ratio is specifically used to help determine if an age group is eligible for transition to the STANDARD_IA (Infrequent Access) storage class.

The ratio is a numerical value (without specific units) that helps evaluate access patterns relative to storage consumption. It's not simply GetRequestCount/Storage_MB as you suggested. Instead, it's a more comprehensive calculation that Amazon S3 uses to evaluate whether objects in a particular age group are being accessed frequently enough to justify keeping them in the STANDARD storage class, or if they would be more cost-effectively stored in STANDARD_IA.

This metric works alongside other metrics in the Storage Class Analysis report such as:

• ObjectAgeForSIATransition: Represents the observed age for transition to STANDARD_IA
• RecommendedObjectAgeForSIATransition: Represents the object age in days to consider for transition after the ObjectAgeForSIATransition stabilizes

Storage Class Analysis observes access patterns for at least 30 days before providing recommendations. The analysis helps you make informed decisions about when to transition less frequently accessed data to lower-cost storage classes, which can be implemented through S3 Lifecycle policies.
Sources
Amazon S3 analytics – Storage Class Analysis - Amazon Simple Storage Service
Configuring storage class analysis - Amazon Simple Storage Service
S3 Storage Analytics and Insights – Amazon S3