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Really? Hrmm. Someone should let all those VPS hosters know that they're doing something impossible. Ceph has block, POSIX file, and S3/object interfaces.

Block storage (RBD) is probably the most widely used interface (judging by mailing list and IRC activity).

Since I asked what the "backend" is, my question is still valid.

It's fundamentally objects, but not S3-style objects. It's more NASD/T10 objects, with rich semantics including partial writes and strong consistency. Implementing block storage on top of this layer (called RADOS) is trivial. Implementing S3-style objects is a bit more work, and implementing a filesystem was downright hard, but both have been done.

Disclaimer: I'm a Gluster developer, which isn't quite the same as being a Ceph developer but we do talk to one another. ;)

/disclaimer/disclosure/ - when you disclose something, it's a disclosure.

Ceph can use RBD (rados block device) to provide block storage.

It's actually both, and a file system.

Oh I guess I thought ceph was all about eventual consistency. I didn't know it was strongly consistent like what I expect from block storage.

Not really. It's object storage with other stuff layered on top of it with varying degrees of success.