ZFS, raidz1: Why is the volume size (42T) so much smaller than the pool size (50T)?

Solution 1:

A 2TB disk is not 2 TiBi in size - it's only 2*10^12 / 2^30 ~ 1862 GiBi.

4 arrays of 6 effective disks each would be 24 * 1862 = 44703 GiBI, or 43.6 TiBi of real, usable storage.

I reckon it has some additional overhead you're not taking into account - IIRC RAIDZ also does snapshots and scrubbing, which take up additional space.

Solution 2:

To clarify the discrepancy in output between the commands:

The zpool command counts the disks that are being used for redundancy as space, while the zfs command does not; thus, the 50.5 TB number is your raw disk size, while the 42T is after taking out the 4 disks for redundancy.