Disk Space Question

[TXAG26]

I am currently setting up a FreeNAS server to be a backup point for a number of computers/devices. It contains 12x 4TB HDD's, in one large RaidZ2. The total gross disk size for the volume is about 41 TB, with about 33TB as shown in FreeNAS that is available to create datasets with.

However, 3.63 TB x 12 HDD's = 43.56 TB, minus 7.26 TB for 2 parity drives = 36.3 TB. Why am I only seeing about 33TB of usable space? Are there any tweaks I can make during volume creation to gain back some of the space? Speed is not a concern, but maximizing space with RaidZ2 is. Any ideas? Ashift settings?

 

[Johnnie Black]

This calculator gives you a good idea of were the space goes, mostly parity, padding and slop space:

https://wintelguy.com/zfs-calc.pl

 

[TXAG26]

This calculator gives you a good idea of were the space goes, mostly parity, padding and slop space:

https://wintelguy.com/zfs-calc.pl

Thank you for the link, I had not seen that calculator before! Very helpful! Unlike other calculators, this one appears to do a great job calculating the "slop", which accounts for the discrepancy I was seeing. Is there anything that can be done to reduce the amount of "slop"?