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.

$ 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.

$# 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