Comment by gregw2

Re: “Reiser3 pioneered journal metadata only”, a metadata-only approach to journaling actually was not uncommon (although for Linux, journaling at all for a time was not available).

This first journalled Unix file system in AIX in the early 90s was metadata only, and faster fsck made it an appealing feature copied by all other commercial Unixes by the late 90s. In the latter 90s, DigitalUnix was the first Unix vendor to provide journaling data, not just metadata. Even SGI’s XFS which had a number of cool and unique features around extent structures optimized for serving video stream large file did not have journaled data.

Most people in Unix then (and now) are unaware of “uncool” operating systems like OS/400 (now iSeries) which arguably delivered on that dream of a melding RAM, ROM, disk, cache. It wasn’t POSIX/Unix/hacker-friendly/cheap but it was and I think still is a billion dollar minicomputer business for IBM. I believe they called it “Single-Level Store Architecture”.

Wikipedia for this for the interested (https://en.wikipedia.org/wiki/IBM_i ): “Instead of operating on memory addresses, TIMI instructions operate on objects. All data in IBM i, such as data files, source code, programs and regions of allocated memory, are encapsulated inside objects managed by the operating system (c.f. the "Everything is a file" model in Unix). …The object model hides whether data is stored in primary, or secondary storage. Instead, the operating system automatically handles the process of retrieving and then storing the changes to permanent storage.”

I believe lisp machines back in the day had something grossly similar.

The difficulty (impossibility?) of implementing this sort of file system in the confines of POSIX (with extensions even?) is probably why this never happened in Unix.

As Reiner’s 2023 letter we are discussing notes, this required new information/hints flowing up and down the layers of the stack. It’d require implementing a new userspace+kernel paradigm of application development alongside the current one.

Accomplishing that sort of transformation in an ecosystem takes a very unique blend of skills and knowledge, and it’s additionally not clear how much economic benefit accrues to anyone for making this shift which is an inhibitor to realization, in Unix, of “the dream”.

But it was done outside of Unix and it quietly is running Fortune 100 workloads today … although many of them are implementing the “strangler pattern” to get those workloads moved to more conventional “modern” systems.