ZFS Metadata On New Server

I have no idea what metadata cache is.
There can be special vdevs storing metadata and small block files only but those are not caches (losing those would break the pool).

I believe that’s what I’m referring to:
ZFS Metadata Special Device: Z there is a thread about it here, the way I understand it is a directory of the ZFS array is stored on the SSD to speed up browsing.

But that would take two of the 3 m.2 connections (assuming I put the Optane in a mirror like suggested above) on the board which doesn’t leave space for both a cache and a boot drive.