I've often wondered why there aren't any (that I'm aware of anyway) Linux hierarchical storage managers (HSM). I realize they are typically used in big iron with things like tape as the base level of storage, but especially when it looked like Optane was going to be a thing it seemed like spinning<->SSD<->Optane<->RAM could be a huge performance opportunity. Though, I guess that's a "layering violation"... Honestly, at this point we've gone almost entirely SSD except for our ZFS backup servers, and that layering violation already has an SSD and RAM cache in it...