Both mysql and pgsql bypass the page cache if they can and maintain their own page caches. You have to do this, otherwise you’re double caching! That is, you’d have your own page cache, which you need to manage calls to read() and to know when to flush pages, while the OS would also have the same pages in its own cache.
(mongodb I believe uses direct mmap access instead of a pagecache, and lmdb does this as well)
(mongodb I believe uses direct mmap access instead of a pagecache, and lmdb does this as well)