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That's S3 Express One Zone. Directory buckets are SSD-backed and regular buckets are HDD-backed. Some of the API differences off the top of my head:

- Directory entries are no longer returned in sorted order in ListObjectsV2

- There's an AppendObject API

- There's a RenameObject API

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It's hard to imagine how these API differences can be explained by the different underlying block device. I don't see any good reason you couldn't support these operations on a HDD.

I suspect it's more to do with the fact that with One Zone is a clean rewrite of large parts of the application stack that makes up S3.

S3 is made up of hundreds of microservices[0], there probably isn't anyone at Amazon that actually understands the whole system. Refactoring it to support these features probably requires coordination between a lot of different teams. They might have petabytes of metadata, making a change to how metadata is persisted probably requires a massive risky data migration.

[0]: "All in, S3 today is composed of hundreds of microservices" - https://www.allthingsdistributed.com/2023/07/building-and-op...

For what it's worth I think you are correct, and that these differences have nothing to do with being SSD-based.

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