Encryption keys are derived from a secret and the hash of the data. The secrets are set per Volume. An empty secret would be a convergent encryption strategy,

Encryption keys are derived from a secret and the hash of the data. The secrets are set per Volume. An empty secret would be a convergent encryption strategy, maybe suitable for public data. If the secret is set then the keys will be convergent with other keys generated with the same secret. This amounts to deduplication within a Volume, and privacy. All a storage provider sees are many small encrypted blobs, so the size of large files is not leaked either.