This is how mongodb originally managed all its data. It used memory mapped files to store the data and let the underlying OS memory management facilities do what they were designed to do. This saved the mongodb devs a ton of complexity in building their own custom cache and let them get to market much faster. The downside is that since virtual memory is shared between processes, other competing processes could potentially mess with your working set (pushing warm data out, etc). The other downside is that since your turning over the management of that “memory” to the OS, you lose fine grained control that can be used to optimize for your specific use case.
Except nowadays with Docker / Kubr you can safely assume the db engine will be the only tenant of a given vm /pod whatever so I think it’s better to let OS do memory management than fight it
