It's not really TTL that's needed here, it's a notion of file/directory tree priority integrated with the filing system APIs. Think about it: there's no reason not to use every single byte of your disk. The only reason we have to leave so much empty space as padding today and manage it so manually is because the OS itself won't automatically delete unimportant files when needed. If it did then apps could just generate caches with abandon safe in the knowledge that they'd be overwritten when necessary. Also, the user would always see the "true" amount of free space, i.e. actual space - caches - tmp files.
