The most interesting finding for me was that you shouldn't run databases on cool filesystems like Btrfs and ZFS, because of COW. So you either use "uncool" filesystems, or disable COW, which is also kinda uncool.
On the other hand, the dumber your file system, the meore r database has over what happens on disk. Thats why databases may allow storing data on disk without using any file system in-between.
