the ISO install does not allow it, as this is a quite non-optimal zfs config. if you configure zfs later (manually via CLI), its possible. but of course, then you cannot use is as root partition.
so best practice, use identical disks.
Actually, I have a use case for booting from a (slightly) mismatched mirror set. It used to be pretty common in some sys-admin circles to use drives from different manufacturers in a raid set (not just drives from different production runs of the same manufacturer).
So if you have two drives of similar but not identical size that you want to use as your mirrored rpool, it's possible by adding the second drive manually after the installation.
The important part is to install on the smallest drive first. During the installation, choose ZFS, Raid0, and the smallest boot drive. Run the installation as usual.
After the system boots up, manually partition the second, larger drive and attach it to your ZFS rpool as a mirror.
One simple way to deal with the partitioning is to run `gdisk <first drive>`, hit 'x' for expert options, 'u' (to replicate partition table), enter the second drive, and if there's an error about the secondary header, just hit 'y' to let it fix it automatically.
If the partition size doesn't match exactly after the fix, it's probably also a good idea to manually resize the partition down to match the first as closely as possible, as this will help in drive replacement down the road. [Size mismatch shouldn't happen if the replication is successful, but it doesn't hurt to check.]
Also probably a good idea to change the GUID of the second disk since the replication seems to copy it from the first drive.
Run grub-install on the second drive, and zpool attach it to the first. Make sure you use the the device name ending in '-part2' which is the default GPT partition used on the first drive.
Appears to work for me. I've manually booted from either drive, but haven't yet tested a failed drive scenario. Don't know if grub will cooperate.
Also of course you should be comfortable manipulating drive partitions and have familiarity with ZFS operations in general. For my part, I'm mostly on FreeBSD so I aside from various Proxmox installs, I haven't booted linux on metal in a long time.
The following thread looks like it could be helpful for ZFS recovery and interacting with grub, should it come to it. But I haven't tested that far myself.
http://forum.proxmox.com/threads/21306-Grub2-recovery-on-ZFS-Proxmox-VE-3-4
As I mentioned, I don't spend much time in linux, so if someone sees any deficiencies in this set up, or have good reasons not to do it this way, I'd be interested in hearing them.
