It only makes sense to treat these as low as possible: arrays of raw memory addresses just like RAM.
That's hardly a low as possible abstraction. For rotating storage it hides bad block remapping. For flash storage it hides wear levelling and delete-before-write.
The search isn't for the lowest possible abstraction to present to the computer, the search is for the abstraction which best mediates between the needs of the computer and the needs of the storage. Storage is increasingly remote and managed, and the current block-based abstraction and your offset-based proposal don't give enough information to the storage's management software.
It's the diversity of storage media that's currently driving object-based storage. It's a lot simpler to build complex storage (with features like migration between flash, disk and tape) if the storage is told what blocks are in a file rather than being left to guess.
Someone asked, why not use a filesystem such as CIFS or NFS? The answer is that this leads to user-based authentication, which leads to a lot of unnecessary complexity for the storage. One of the aims of OBS is to allow storage to be leased out, and integrating with customers' authentication systems would have introduced a big hurdle.
Please note that I'm not a OBS defender, I'm only seeking to explain it. Conversely, I'm also not saying that OBS is such a poor idea that it shouldn't be in Linux. My own view is that the SCSI protocol itself is now inadequate for enterprise storage, as it is a poor fit for the link, network and transport protocols used in corporate networks. I don't see much sense in using a disk protocol to communicate between a computer and a storage manager (ie, another computer). There's a lot of pretence happening there which could be stripped away for better performance and robustness. My view may be overly coloured by experience as a participant in the iSCSI working group
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