Appears to be about correct, your raw space is 2 x 3TB + 2TB = 8TB (7.45 real TB, RAW). Now you have used different sized disks so your pool is a multiple of the smallest drive, 2TB, so 6TB (5.58TB), RAW. To use all your disk space you would have to partition the larger drives so they had a 1 and 2 TB partition and make 2 pools, 1 raidz made of 3 x 2TB and a second mirror of 2 x 1TB.

This would give you 4TB usable on the raidz pool and 1TB on the mirror.

Wait, doesn't 2 x 3TB + 2 x 2TB = 10? From what I've read with RaidZ you lose the biggest drive to parity so should my pool be 10 - 3 or 7?

You said you have "2 TB and 2 3TB SATA disks in RaidZ1"

So 1 x 2TB and 2 x 3TB, but you say you actually have 4 drives total?

Parity isn't stored on one drive its stored on all of them, the size of the smallest drive x the number if drives - 1 is the usable space for raidz1

Yes sorry, I made a typo. I have 4 drives, 2x 2TB and 2 x 3TB. So if I understand you correctly, for ever 3TB drive I add to this array I will essentially lose 1TB of storage?

I guess I should look into getting more drives and creating 2 arrays, one with all 2TB drives and one with all 3TB drives.

I don't have any data on these drives that I'm worried about yet, but if I were to replace the 3TBs one by one with 2TB drive, will the array rebuilt?

Thank you,

Correct, unless you partition and create a second pool with the remaining space on those disks.

Getting more drives would mean you would have less wasted space, with a single pool.

Not sure, but I dont see why not (espectially if it is partitioned), as it wouldnt need to shrink the pool, but not 100% sure on that. could test on a vm.