The BDB mmmap cache actually worked fine for many years, on older OSs. They didn't keep accurate track of which memory pages were dirtied or when, so they were

The BDB mmmap cache actually worked fine for many years, on older OSs. They didn't keep accurate track of which memory pages were dirtied or when, so they were pretty lazy about flushing changes back to disk. This made an mmap'd file functionally equivalent to a SysV shared memory region. I remember when Linux changed in the 2.6 series, and suddenly using an mmap'd file got incredibly slow, because the kernel was proactively flushing dirtied pages back to disk. We started recommending SysV shm for all our deployments after that. You had to be careful in configuring this, since SysV shm wasn't swappable or pageable.