To recap my understanding of options when wanting to Expand a 2-Drives setup:
Option 1: Expand to RAID10
Add 2 new drives as a second mirror pair (zvol) to your pool, creating RAID10.
Option 2: Rebuild as RAIDZ
Recreate the pool as RAIDZ1, RAIDZ2, or RAIDZ3. Note: RAIDZ1 can be done with only 3 drives, so you may only need to buy one additional drive.
RAID10
RAIDZ
Backup Required?
No
Yes
Ease of Implementation
Easy
Complex (requires pool recreation)
Usable Storage
50% of total capacity
67%+ (with RAIDZ1 on 3 drives)
Redundancy
High (tolerates 1 drive failure per mirror)
Varies (1–3 drives depending on RAIDZ level)
Performance
Faster (striping + mirroring)
Slower (parity calculations)
Cost
Requires 2 additional drives
Can start with only 1 extra drive (RAIDZ1)