NOOB: Where did my available storage go?

[csfuser]

Searched the forum+documentation but can't find the answer.
FreeNAS noob... home user, not looking for massive complexity, raid storage, striping or fancy configuration.
Running v11.3-u3.1 on an Intel i7-7600k, 8GB RAM box.
Installed OS on 16GB USB drive, using 8TB internal drive and 4TB external USB3 storage.
Both drives allocated to same pool.

Individual disks show available space as 7.28 and 3.64 TB or [theoretically] 10.92 TB total.
Pool stats show available space as 10.54 TB..

So where did all my space go..?? I'm missing 0.38G (between disks and poll nbrs).. and almost 1.5TB overall (yes, I know disks cannot be fully allocated).
How is it being calculated..
Can someone share their expertise on where all those GIGS of space are vanishing or is it all based on how the OS calculates it... ??

Thnaks

 

[Samuel Tai]

This is the usual misunderstanding between base-10 and base-2. The drive manufacturers typically use base-10 (metric T is 10^12), because the drives appear larger with these units. However, computers operate on base-2, so typically operate using Tebibytes (2^40). 8 TB (metric) is 7.28 TB (bi). 4 TB (metric) is 3.64 TB (bi). No space has been lost, only a different unit applied. (Now you see why the manufacturers use metric units.)

You're correct that 0.38 TB (bi) are unavailable. Some of that is reserved for the GPT partition tables and FreeNAS swap space on each drive, and the majority of that is used for ZFS file system data structures.

 

[csfuser]

This is the usual misunderstanding between base-10 and base-2. The drive manufacturers typically use base-10 (metric T is 10^12), because the drives appear larger with these units. However, computers operate on base-2, so typically operate using Tebibytes (2^40). 8 TB (metric) is 7.28 TB (bi). 4 TB (metric) is 3.64 TB (bi). No space has been lost, only a different unit applied. (Now you see why the manufacturers use metric units.)

You're correct that 0.38 TB (bi) are unavailable. Some of that is reserved for the GPT partition tables and FreeNAS swap space on each drive, and the majority of that is used for ZFS file system data structures.

Makers perfect sense.. Thanks for the explanation.. !