I worked for 5 years on VMS for a real-time satellite image processing HW/SW system for NASA. VMS was wonderful. At another company, I then worked on a Unix system for a few years, then 2 years on a port of our software to Unix. (The port was smooth and quick, thanks to VAX C and its relatively complete library; most of the 2 years was spent adding new capabilities for a particular customer.) The rest of my career has mostly been under Unix.

Microsoft hired the main architect of VMS, Dave Cutler, away from DEC to design Windows NT. (VMS++ = WNT!) I haven't worked on any in-depth Windows programming projects. I did read an article some years ago about Cutler, VMS, and WNT. The author pointed out an insightful distinction between VMS and Unix with regard to I/O operations: VMS tells you when an operation completes, whereas Unix tells you when you can begin an operation. As a result, I think, asynchronous I/O under VMS was there and available from the git-go, but always seemed like some odd thing grafted onto Unix, making it more painful than it should have been. I have never used it, but I believe WNT's overlapped I/O was patterned after the VMS model?

> Microsoft hired the main architect of VMS, Dave Cutler, away from DEC to design Windows NT.

Not sure why you mice words. Cutler developed NT at Digital. That's better. In 1988, Cutler took his work (the literal data, the MICA OS code from DEC's cancelled Prism RISC project) and his engineering team at DEC with him to Microsoft. I believe it was much less like poaching and much more like defecting. DEC ultimately forced Microsoft into an alliance under threat of lawsuit for the IP theft to migrate the VMS user base to NT, with Microsoft paying quite a lot, $180M, to avoid lawsuit, for, get this... training DEC engineers to use NT. In 1995, someone at MIT found large chunks of DEC MICA code unaltered, including comments, in Windows NT.

There is a book that addresses the reasons for Cutler's departure from DEC, but it does not go into the details that you have mentioned.

https://www.goodreads.com/en/book/show/1416925.Show_Stopper_

But then either VMS is much closer to UNIX than to things like IBM i (or IBM mainframes, which are however themselves very, very different from IBM i), or Windows is closer to UNIX than VMS. Because Windows is very close to UNIX compared to either of those.

> But then either VMS is much closer to UNIX than to things like IBM i

I'm not really sure that is the case at all. VMS had versioning file system that had support for multiple file types including stream, sequential, indexed and relative. It had a very different security model, four levels of processor access (unix there's kernel and userland) and it had very different networking capabilities, including clustering baked in (in 1983).

> Because Windows is very close to UNIX compared to either of those.

There's deep VMS heritage in Windows NT: both VMS and NT were written by David Cutler.

I know! But then VMS is maybe closer than it seems? Have a look at IBM i.

Agreed. I've only limited experience with i, but more with z. After using them, the other OSes you might use start to look and feel pretty similar to each other.

Microsoft hired the main architect of VMS, Dave Cutler, away from DEC to design Windows NT. (VMS++ = WNT)

You've got it backwards. Digital cancelled Cutler's pet project and he shopped himself, and Digital's IP, to Microsoft. VMS and NT have a lot of similarities, but NT is not VMS, though they have a common ancestor: MICA = NT

[0] https://en.wikipedia.org/wiki/DEC_MICA

Windows has very stringent file locking; Unix doesn't.

I believe this major design difference came from VMS.