What Mr_N said is good advice.
Basic ZFS primer: Pools are made of 1 or more vDevs. VDevs are made of physical drives. Once a Zx vDev is created, it cannot be made "larger" in capacity. If the vDev is a mirror, you can add drives to it to increase the parity count. Pools capacity can be increased by adding additional vDevs. It is recommended that all vDevs in a pool be made of the same RAID level using the same type of drives (capacity, RPM, block size). ZFS will allow you to mix vDev types in a pool if you want to, but, just because you can does not mean you should. :)
Note: if anyone finds mistakes above, please let me know.
I run Z2 in a 2x6 array of 12 drives. Fast enough for my needs. Resilient too as I have experienced a number of failed drives in the last 3 years due to Seagate 3TB drives being shite without data loss or unavailability. I have never needed to use my backups which is convenient for me.
RaidZ1 is not as resilient to failure as RaidZ2 (or mirroring) because failures during a resilver (rebuilding of a ZFS pool) will cause the loss of the pool and all of the data on it. Mirrors are faster than Z2 but offer 50% space efficiency whereas Z2 can offer 50% (4 drives) up to 80% (10 disks, basically the max number you should use in a Z2 vDev from what I have read). Also, and this really gets into complicated probability functions, Z2 arrays, if configured properly, are more resilient to failures than mirrored sets if a large number of drives are involved. This is because if 2 of 2 drives in the same mirror fail, then the entire pool is lost whereas a Z2 pool can tolerate 2 drive failures without data loss regardless of which 2 drives fail. And to complicate things, you can make the mirrors out of as many drives as you want i.e. 3 or 4 drives mirrored to make each vDev. Sacrificing space for availability. Fun times.
You, unfortunately, have run into the 1 thing that ZFS does not do that would be really nice if it did. You have to plan ahead with ZFS. I would have preferred to use a 2x8TB Z2 pool arrangement but I did not have enough drive bays available to do so in my Norco case, and I have 22 drives in it :) Moving forward, I will always sacrifice 33% capacity to parity and need to add 6 additional drives if I want to expand my pool while following ZFS best practices.
Another item to consider is that any form of RAID (5, 6, 50, 60, Z1-Z3, mirrors) will NEVER replace backups (nothing personal, I harp about this all the time :)). RaidZ2 will be far more tolerant to failure than Z1 (or no RAID at all) but please consider backing up your irreplaceable data regardless of RAID level.
Cheers,
