ZFS reported space vs used space

[Velixa]

Hi all, I'll ask help here as my googling just made me more confused than before.
I have a zpool raid5 of 18TB x 3 HDD. In theory, accounting for redundancy, there should be 36TB available and that's correct.
The issue is that when allocating a VM disk it allows me to go as far as 26TB, and when it's done creating it the 26TB image will take the whole reported 36TB. I assumed that it was an incorrect TiB vs TB report but 36TB is 32TB, and even accounting for filesystem and stuff it's still missing 6 or more TBs.
I'll attach both VM Disk screenshot and Pool usage. Can you help me understand if there is some overhead i'm missing? Thanks in advance for your help.

 

[LnxBil]

I have a zpool raid5 of 18TB x 3 HDD. In theory, accounting for redundancy, there should be 36TB available and that's correct.

In RAID5 it's true, but there is no RAID5 in ZFS. RAIDz1 is no RAID5 and that's the wrong assumption.

Can you help me understand if there is some overhead i'm missing?

Yes, the padding overhead and best volblocksize for a given set of disks. Just search again with this and it also has been discussed plenty of times

 

[Dunuin]

Capacity loss for a 3-disk raidz1 with ashift=12 would be 50% at volblocksize 4K/8K or 33% for volblocksize >= 16K. So set the "Block size" field of the ZFSpool storage to 16K, destroy all VM virtual disks and recreate them.

Also keep in mind that while it is possible to use the full space you shouldn't fill the pool more than 80% because otherwise it will get slower and fragment faster. So thats actually only 28,6T of usable space.