This feature is poorly supported, and causes grief for cache development due to how it #ifdef’s with the system. There are plans for a new system that would make the concept of cache hierarchies generic. In the mean time, the suggestion is that if you need this, you use one of
1) Assigning content to volumes and disks via our existing configs.
2) Use Linux’s bcache feature, which comes with Linux 3.10 or later. I suspect e.g. ZFS has something similar.
3) Use a third party system to implement the hybrid device (e.g. flash cache or EnhanceIO.