> Page management isn't really a thing we can do well "in user space".
But it is the thing most high performance OLTP DBMSs, most of us are aware of, do? I'm also not sure your cite is relevant here. Or it is at least niche. The comparison is made to LeanStore, which is AFAICT is not feature complete, and a research prototype?
Your cite does not describe a unikernel use case, but instead in kernel helper modules. Your cite is about leveraging in kernel virtual memory for the DB buffer cache, and thus one wonders how sophisticated the VM subsystem is in most unikernels? That is -- how is this argument for unikernels? Seems as though your cite is making the opposite argument -- for a more complex relationship between the DB and the kernel, not a pared down one.
> And then, further, a DB goes and basically implements its own equivalent of a filesystem, managing its own storage. Often fighting with the OS about the semantics of fsync/durability, etc.
The fights you're describing what have thus far been the problems of ceding control of the buffer cache to the kernel, via mmap, especially re: transactional safety.
If your argument is kernels may need a bottom up redesign to make ideas like this work, I suppose that makes sense. However, again, I'm not sure that makes unikernels more of an answer here than anywhere else, though.
> I don't think it's an unreasonable mental leap for people to start thinking: "I'm by necessity [cuz cloud] in a VM. Now I'm inside an OS in a VM, and the OS is sometimes getting in my way, and I'm doing things to get around the OS... Why?"
I think that's a fair thought to have, but the problem is how it actually works in practice. As in, less code seems really enticing, the problem is what abstractions are you throwing away. If the abstraction is less memory protection, maybe this is not a good tradeoff in practice.