Volume size difference

[Johannez]

today i made another volume and found a difference i cant explain.

In above picture the estimated size of 8x4TB in RaidZ2 will be 21.82Tb
After creating the volume i got 20Tb exactly:

If i check a raid calculator to verify my own calculation i get the same result, 21.82Tb
What makes it weird it is exactly 20Tb.

What am i missing?

 

[SweetAndLow]

This is pretty normal, there are threads on there talking about filesystem layout with different drive counts in a raidz2. 6 is pretty efficient and 8 is a little worse but nothing really to worry about.

 

[Johannez]

I'm sorry but i don't understand.

Is it a graphical error and is the size 21.8tb? or is it a calculation error and does a raidz2 with 8 drives take up more for parity?
Just strange but it is exactly 20tb like it is some cap (i read volumes can be much bigger so that won't be it).

It's not really the storage i miss but i would like to know the "why" and "where" it is used for.

 

[Bidule0hm]

This thread and this other thread may answer your questions ;)

 

[Johannez]

Thank you Bidule0hm this indeed answers my question.