DM and MD come a little closer

Filesystem internal RAID implementations shouldn't count on the list. A handful of minor utility routines aside such implementations aren't candidates for general use at all. They are completely different in character, draw their advantages due to filesystem level knowledge of what is going on at a very low level, and are more or less completely unfactorizable.

It is certainly conceivable that an interface could be developed to speed RAID recovery of a disk containing conventional filesystems by indicating which sectors are actually in use (couldn't TRIM be used for that?), but the problem is, like flash block devices, a modern RAID implementation is rapidly taking on more of the character of a filesystem itself, in terms of the complexity of on disk data structures, journalling, metadata etc. And it often isn't particularly efficient to run two such implementations on top of each other, even though often convenient in practice.