Bcache: Caching beyond just RAM

There's no way to do that now - all the cache devices in the system are pooled together and used symmetrically - but it is something I've been considering down the road. It might not be all that much more work, I'm just curious if the performance would justify it. Most people's working set just isn't all that huge, and also the big problem with your big slow raid6 is random writes, which bcache should be able to mostly eliminate. But we'll see.