Significant Loss Of Storage Space?

[{c}guy]

Please don't kill the newbie to FreeNas, but I've searched quite a bit and can't seem to find a definitive answer...
I've got 6 x 6tb WD Red NAS drives installed for a total space of 36TB of storage, but when I configured my pool using Raidz2, I ended up with only about 20TB of storage?
I've read all about 2 disks being used for parity, so take away 12tb and I should still have 24tb left... not 20.. I'm missing almost a whole 'nother disk!

Given that I'm a noob I figured I'd better ask someone before I get to far down the road only to realize that I screwed something up in my config.

Here's the rest of my hardware specs:

2x m1015 flashed with IT firmware
supermicro x9srl-f board
intel xeon e5-2660 v2
64 GG dd3 ram

Thanks in advance for any insights you can provide.

 

[Basil Hendroff]

I think you'll find this calculator useful. It looks like you're in the ballpark. https://www.ixsystems.com/community/threads/zfs-drive-size-and-cost-comparison-spreadsheet.38092/

 

[{c}guy]

Thank you. Yes., that's exactly what I needed to know. It does indeed look like I'm right in the ballpark. There must be some overhead that accounts for the extra 4tb loss.

 

[SweetAndLow]

That's just how hard drives work. There is the TB to TiB conversion and then a little zfs overhear but that is not noticable.

 

[Chris Moore]

I've read all about 2 disks being used for parity, so take away 12tb and I should still have 24tb left... not 20.. I'm missing almost a whole 'nother disk!

Here is a link to a useful calculator tool:
https://wintelguy.com/zfs-calc.pl

 

[Basil Hendroff]

Here is a link to a useful calculator tool:
https://wintelguy.com/zfs-calc.pl

Worth including in the Useful Threads resource?

 

[Ericloewe]

Quick estimate of "loss" between TB and TiB is ~0.909. It's a useful number to keep in mind.