storage space used in 'empty' UFS 2 way mirror in FreeNas 9.2.0 x64

[dominator99]

Have checked forums & can't find an explanation for the large amount of space used (72GB) in a 2 way UFS mirror using 2 x 1TB HDD's.

Freenas storage lists 8.0Kib (8%) used -- 830.1GiB available -- 902.3GiB size for the UFS mirror named M3

My other 2 UFS mirrors (M1 & M2) list used in GiB units but neither is empty & both are 3 way UFS mirrors (using 3 HDD's in each mirror).

Can anyone explain such a high overhead (72GB) for a UFS 2 way mirror?
Also, 8% of 902.3GB is 72.2GiB so what is the 8.0KiB figure?

 

[gpsguy]

By default, 8% is set aside for the reallocation of defective sectors. You can change the size - read more about the subject in this thread.

 

[dominator99]

By default, 8% is set aside for the reallocation of defective sectors. You can change the size - read more about the subject in this thread.

Thanks for the info