Update documentation for S3 Express One Zone

[monthonk]

Description of change

Update the configuration and semantics documents to address differences on using Mountpoint with S3 Express One Zone buckets.

Relevant issues: #651

Does this change impact existing behavior?

No, only document updates.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and I agree to the terms of the Developer Certificate of Origin (DCO).

[arsh]

Is this something that could be enforced by code instead of relying on this documentation?

[monthonk]

This value is still valid for directory buckets, but it's the same as not setting storage class at all. So, maybe we can just don't allow people to set --storage-class to EXPRESS_ONEZONE.

[monthonk]

Actually, users also cannot set storage class other than EXPRESS_ONEZONE for directory buckets. I think the right time to do the validation is after we have resolved the endpoint and know whether this is a directory bucket or not.

Let's do that in a separate PR.

[dannycjones]

FWIW, I'd caution against setting limits like this that would normally be enforced service-side in S3 but could feasibly be relaxed in the future. In adding this restriction to the client, users are forced to upgrade version to adopt new features (like a new storage class for directory buckets).

Maybe forcing updates has its upsides from maintainers' point of view, but I'd lean against it for now. 😄

[dannycjones]

What's important is that the user gets clear feedback into what went wrong. If the wrong storage class is set, is the error clear in the Mountpoint logs? Of course we're always limited by the error responses we can send via filesystem APIs.

[monthonk]

Currently, users will get InvalidStorageClass in the logs. But, they only get it on the first write which could be pretty late. I think at least we should do the validation on mount and output a warning message so that they could check their storage class configuration.

[dannycjones]

Thanks Monthon, just a bit more feedback. Most is inline.

On storage classes, can we update the main body to be a bit clearer on default behavior. Mountpoint doesn't specify a storage class and respects the default from S3 - which is STANDARD for general purpose buckets or EXPRESS_ONEZONE for directory buckets.

- By default, Mountpoint uses the S3 Standard storage class, which is appropriate for a wide variety of use cases.
+ Mountpoint respects the default storage class from S3 unless otherwise configured, which is appropriate for a wide variety of use cases.

[dannycjones]

LGTM, thanks @monthonk.