I'm seeing 10TB more space than I expect in my raidz1 array

[komali2]

I have a funky array of 14, 14, 18, 18 TB drives. I set them up as raidz1:

sudo zpool create -f media raidz1 (drive 1, 2, 3, 4)

I got the error about drives being different size, and overroad it with -f. Now, I'm expecting at best 40TB of space, based on a bunch of random calculators I tried out (I mean in head math I don't see why I'd get more than 42 but this is black magic to me so whatever). Yet if I do zpool list , I see:

NAME     SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
media   50.9T   744K  50.9T        -         -     0%     0%  1.00x    ONLINE  -

That's only 8TB less than when I had this monster in a stripe. What gives?

[rincebrain]

zpool list includes the space occupied by parity overhead in its space, because (among other reasons) the ratio of parity overhead to space used isn't fixed.

zfs list (and df) give an estimate (because, again, things can be less efficient if you're storing smaller things) of space after that overhead.

You may find this interesting reading.