PowerScale OneFS: Interpreting reported disk capacity in tools such as isi status and df

Storage capacity on a PowerScale cluster is measured using base 2 storage units, for example tebibytes, TiBs, or equivalent.
Output of 'isi status' on the cluster the capacity is shown in TiBs.
Generally, disk manufacturers use base 10, for example terabytes/TBs and this would account for a discrepancy which may have been seen. 

1 tebibyte = 2^40 bytes  = 1099511627776 bytes
1 terabyte = 10^12 bytes = 1000000000000 bytes

Isilon uses 2^10 = 1024 bytes. 1024^4 = 1 TiB (Tebibyte), where disk manufacturers use 10^3 = 1000 bytes. 1000^4 = 1 TB (Terabyte)

To calculate manually:

1) Find total raw TB in base 10 (Disk manufacturer capacity)

2) Multiply that result by (1000^4/1024^4) to get the base 2 value.

For example, if 1) = 66 TB, then '66 x (1000^4/1024^4) = 60.0267 TiB'

Related commands on the cluster:

'df -H /ifs'  shows size and usage in decimal units, 1 KB = 1000 Bytes

'df -h /ifs' shows size and usage based on powers of 1024, 1 KiB = 1024 Bytes

Finally, there is an overhead on Disk Pools of less than 0.8% per disk.
This essentially means that the cumulative block count per disk would show higher than that reported by the Disk Pool.
As the cluster gathers disk capacity info from DiskPools there is a further disconnect between the expected and actual capacity of each disk within the cluster.
The larger the cluster size the more apparent this overhead becomes.