You're doing it wrong.

ZFS does support mixed spinning and silicon storage in a single pool, but in a different way than you're attempting. Instead of manually managing which data goes where, ZFS wants to handle this for you via its ZIL and L2ARC caches.

Set up this way, ZFS will figure out what needs to be where based on your usage patterns.

You're doing it wrong.

ZFS does support mixed spinning and silicon storage in a single pool, but in a different way than you're attempting. Instead of manually managing which data goes where, ZFS wants to handle this for you via its ZIL and L2ARC caches.

Set up this way, ZFS will figure out what needs to be where based on your usage patterns.

If you must have manual management over what goes where, I'd suggest simply making three pools: one each for the SSDs, and a mirror or RAID-Z for the HDDs. Use one SSD pool for the root filesystem, one for /home, and the slow HDD pool for bulk data. Trying to squish it all into a single thing yet still maintaining manual management is mixing concepts. Either you want ZFS to manage which disks your data lives on or you do not.