I am hopeful the following will help someone scratch their head a little less in trying to understand the info returned by zfs.

I set up a pool using 4 2TB SATA disks.

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$ zpool list -v
NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
rpool 7.25T 2.50T 4.75T - 10% 34% 1.00x ONLINE -
raidz2 7.25T 2.50T 4.75T - 10% 34%
sda2 - - - - - -
sdb2 - - - - - -
sdc2 - - - - - -
sdd2 - - - - - -

The total size displayed here is the total size of the 4 disks. The maths works as 4*2TB = 8TB = ~7.25TiB

RAIDZ2 is like RAID6 and it uses two disks for parity. Thus, I would expect to have ~4TB or 3.63TiB of available space. I haven’t been able to find this number displayed anywhere.

However, you can find the amount of disk space still available using the following command.

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$# zfs list
NAME USED AVAIL REFER MOUNTPOINT
rpool 1.21T 2.19T 140K /rpool
rpool/ROOT 46.5G 2.19T 140K /rpool/ROOT
rpool/ROOT/pve-1 46.5G 2.19T 46.5G /
rpool/data 1.16T 2.19T 140K /rpool/data
rpool/data/vm-100-disk-1 593M 2.19T 593M -
rpool/data/vm-101-disk-1 87.1G 2.19T 87.1G -
rpool/data/vm-102-disk-1 71.2G 2.19T 71.2G -
rpool/data/vm-103-disk-1 2.26G 2.19T 2.26G -
rpool/data/vm-103-disk-2 13.2M 2.19T 13.2M -
rpool/data/vm-103-disk-3 13.2M 2.19T 13.2M -
rpool/data/vm-103-disk-4 93K 2.19T 93K -
rpool/data/vm-103-disk-5 1015G 2.19T 1015G -
rpool/data/vm-104-disk-1 4.73G 2.19T 4.73G -
rpool/data/vm-105-disk-1 4.16G 2.19T 4.16G -
rpool/swap 8.66G 2.19T 8.66G -

The value of 2.19T is the amount of unallocated space available in the pool. To verify this, you can run

# zfs get all rpool
NAME PROPERTY     VALUE                           SOURCE
rpool type        filesystem                       -
rpool creation    Fri Aug 4 20:39 2017             -
rpool used        1.21T                            -
rpool available   2.19T                            -

...

If we add the two numbers here, 1.21T + 2.19T = 3.4T.

5% of disk space is reserved, so 3.63 * 0.95 = 3.4T

et voila