Using a zpool mirror; is there a way to set something like a primary storage / preferred vdevs / read-first devices like it was in disksuite using "metaparam –r [roundrobin | geometric | first]" or the "vxvol rdpol [prefer | round | select | siteread]" to define a preferred plex in VxVM? Will ZFS always use round-robin?
In our example we want to use a brand-new fast storage with hot-data tiering (ssd-caching) mirrored/combined to an older/slower one... with round-robin we could assume that every second read will be slower?
Will the write acknowledgement be commited after both writes so we will have no advantage from the faster storage?
Yes, this problem is understood and a bug is filed (15466549).
Writes will go to both sides of the mirror and will wait for acknowledgement. Reads will alternate.
If possible, I would put your most important data or data that needs the best performance, in a pool on the faster new devices.
This would mean keeping your slower old disks in one pool and new, faster disks in another pool.
Another idea is to use the SSDs as either separate cache or log devices, depending on whether you need faster read or faster write performance.