Re: Anything we should steal ? - Definitely
"Like OS/400 and AIX?"
OS/400 (now "IBM i") is the other commonly cited OS that provides a single level store, and AFAIK the only one still in active commercial use today. (Aegis and KeyKOS are two other, now defunct, examples that I remember OTTOMH.)
AIX OTOH is IBM's homegrown (SysV-based) implementation of Unix and while certainly providing mmap(2) & friends I am not aware that it has a single level store. Not that it would be impossible to implement a POSIX-ish OS on top of an OS based on a SLS (it has, in fact, been done: Domain/OS), but to my knowledge AIX is basically SysV with the usual set of BSD extensions as well as IBM's very own bells and whistles added.
Note that a "single level store" means more than just the ability to mmap files (I had mentioned mmap only as a /very loose/ Unix-analogy); in fact such a system does not have a "file system" in the usual sense of the word at all - just a large, usually segmented, address space or object store that is transparently being made persistent by the OS through writing out dirty pages to permanent background storage as needed.
No need for file I/O to keep data persistent, /that/ is the key point.
FWIW, the concept appears not to be entirely "dead"; while searching Multics references I stumbled on this paper
but I have not yet had time to really digest it. And then there is of course "The Machine", or rather, with quite a bit of luck there will be one day... although "memory centric computing" - while also being built around a persistent main memory is a different and more far-reaching concept altoghether.
BTW, another Multics nicety that I had forgotten in my original post is the fact that Multics segements containing executable code could (being mapped into the address space all the time anyway) be directly invoked and the interface for doing so was IIRC provided and standardized by the OS, thus leading to implicit cross-language compatibility of executable code. No need for FFIs and stuff.