The simple answer is that to mirror something takes almost no processing power - it just writes to the disk a second time. For RAID-Z2, you have to compute an entirely new parity block, which although small CAN bog down the CPU when you have to write large amounts of data quickly.
Mirroring is always the preferred solution for high-speed data, if it's just bulk-storage without fast write speeds, RAID-Z2 is a good alternative that does allow any two drives to die as you allude to.
The other advantage is that mirrored pools can be expanded with more mirrored devices - while a RAID-Z2 can not be expanded - though more RAID-Z2 storage can be added to the pool, it will be two RAID-Z2 storage pools concatenated (in effect) rather than equally split between all the storage and striped.
With 20 disks you have a lot of options. I'm assuming you already have drives for the OS, so the 20 disks would be dedicated data drives. In my Sun Fire x4540 (48 drives), I've allocated 20 drives in a mirrored setup and 24 in a striped raidz1 config (6 disks per raidz and 4 striped vdevs). Two disks are for the OS and the remainder are spares.
Which controller are you using? You may want to refer to: ZFS SAS/SATA controller recommendations
Don't use the hardware raid if you can. ZFS thrives when drives are presented as raw disks to the OS.
Your raidz1 performance increases with the number of stripes across raidz1 groups. With 20 disks, you could use 4 raidz1 groups consisting of 5 disks each, or 5 groups of 4 disks. Performance on the latter will be better. Your fault tolerance in that setup would be sustaining the failure of 1 disk per group (e.g., potentially 4 or 5 disks could fail under the right conditions).
The read speed from a raidz1 or raidz2 group is equivalent to the read speed of one disk. With the above setup, your theoretical max read speeds would be equivalent to that of 4 or 5 disks (for each vdev/group of raidz1 disks).
Going with the mirrored setup would maximize speed, but you will run into the bandwidth limitations of your controller at that point. You may not need that type of speed, so I'd suggest a combination of raidz1 and stripes. In that case, you could sustain one failed disk per mirrored pair (e.g. 10 disks could possibly fail if they're the right ones).
Either way, you should consider a hot-spare arrangement no matter which solution you go with. Perhaps 18 disks in a mirrored arrangement with 2 hot-spares or a 3-stripe 6-disk raidz1 with 2 hot-spares...
When I built my first ZFS setup, I used this note from Sun to help understand RAID level performance...
http://blogs.oracle.com/relling/entry/zfs_raid_recommendations_space_performance
Examples with 20 disks:
20-disk mirrored pairs.
pool: vol1
state: ONLINE
scrub: scrub completed after 3h16m with 0 errors on Fri Nov 26 09:45:54 2010
config:
NAME STATE READ WRITE CKSUM
vol1 ONLINE 0 0 0
mirror ONLINE 0 0 0
c4t1d0 ONLINE 0 0 0
c5t1d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c6t1d0 ONLINE 0 0 0
c7t1d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c8t1d0 ONLINE 0 0 0
c9t1d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c4t2d0 ONLINE 0 0 0
c5t2d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c6t2d0 ONLINE 0 0 0
c7t2d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c8t2d0 ONLINE 0 0 0
c9t2d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c4t3d0 ONLINE 0 0 0
c5t3d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c6t3d0 ONLINE 0 0 0
c7t3d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c8t3d0 ONLINE 0 0 0
c9t3d0 ONLINE 0 0 0
mirror ONLINE 0 0 0
c4t4d0 ONLINE 0 0 0
c5t4d0 ONLINE 0 0 0
20-disk striped raidz1 consisting of 4 stripes of 5-disk raidz1 vdevs.
pool: vol1
state: ONLINE
scrub: scrub completed after 14h38m with 0 errors on Fri Nov 26 21:07:53 2010
config:
NAME STATE READ WRITE CKSUM
vol1 ONLINE 0 0 0
raidz1 ONLINE 0 0 0
c6t4d0 ONLINE 0 0 0
c7t4d0 ONLINE 0 0 0
c8t4d0 ONLINE 0 0 0
c9t4d0 ONLINE 0 0 0
c4t5d0 ONLINE 0 0 0
raidz1 ONLINE 0 0 0
c6t5d0 ONLINE 0 0 0
c7t5d0 ONLINE 0 0 0
c8t5d0 ONLINE 0 0 0
c9t5d0 ONLINE 0 0 0
c4t6d0 ONLINE 0 0 0
raidz1 ONLINE 0 0 0
c6t6d0 ONLINE 0 0 0
c7t6d0 ONLINE 0 0 0
c8t6d0 ONLINE 0 0 0
c9t6d0 ONLINE 0 0 0
c4t7d0 ONLINE 0 0 0
raidz1 ONLINE 0 0 0
c6t7d0 ONLINE 0 0 0
c7t7d0 ONLINE 0 0 0
c8t7d0 ONLINE 0 0 0
c9t7d0 ONLINE 0 0 0
c6t0d0 ONLINE 0 0 0
Edit:
Or if you want two pools of storage, you could break your 20 disks into two groups:
10 disks in mirrored pairs (5 per controller).
AND
3 stripes of 3-disk raidz1 groups
AND
1 global spare...
That gives you both types of storage, good redundancy, a spare drive, and you can test the performance of each pool back-to-back.
Best Answer
Mirroring is the way to go here. It'll let you easily add additional mirrored pairs to extend your existing volume. No downtime, no tricks, just add more disks and you're on your way. Unless you really need the 6TB of 4x2TB in RAIDZ, the 4TB of 4x2TB mirrored is a better bet. Mirroring also has the benefit of lowered CPU overhead, better performance and easier recovery in case of failure.
But seeing as people prefer to live on the edge and save a couple bucks here's how migrate your data from a two drive mirror to a four drive raidz setup:
zpool create mirror cXt1d0 cXt2d0 yourPool
zpool detach cXt2d0 yourPool
mkfile -n 500GB /path/file.img
zpool create raidz cXt2d0 cXt3d0 cXt4d0 /path/file.img newPool
zpool detach /path/file.img newPool
zpool destroy yourPool
zpool attach cXt1d0 newPool