The problem with filesystems is that they don't usually support transactions. For example, it is not possible to atomically change two files, without resorting to ugly and dangerous hacks.
Plan 9 came close. You could create /foo/a and /foo/b at your leisure, then bind /foo over /etc. This atomically replaced /etc/a and /etc/b with /foo/a and /foo/b, for the purposes of open. The change could be atomically reverted with unmount.
If that wasn't good enough, then /etc/a and /etc/b could be virtual files served by the same daemon. This could look for change sets written to /dev/patch, and block reads and writes while it applied them.
Right. If you're changing the configuration of a live system, you often want to do it atomically. Databases are designed for that. They also support search operations and have indexes, which get updated as the data changes.
