You can try, but it's not going to be very consistent. You're at the mercy of whenever the OS decides to start flushing pages.. head of line blocking, mmap_sem locks, file descriptor locks, etc. My old job had an mmap-on-disk storage engine at scale and it wasn't any fun.

I think the worst part is mmap-on-disk looks fine at first, and only comes out as a problem after you scale up a while. False sense of security. :/