Clear as mud:
Re: How do you calculate usable capacity for Isilon?
Here is the general accepted formula used when sizing:
1) Find total raw TB in base 10
2) Multiply that result by (1000^4/1024^4) to get base 2 TB
3) Subtract 1 GB per drive for the OS partitions
4) Subtract 0.0083% of that result to account for the filesystem format
5) Subtract the protection overhead from that result
As for the protection overhead that you are planning to use, look to the "OneFS User Guide" on support.emc.com. Skip to the section: "OneFS data protection" where it will talk about N+M data protection, protection schemes such as N+1, N+2:1 (default), 2x, etc and the associated cost/parity overhead. Also, you will see a very good matrix listing the percent overhead which begins by reminding us: "The parity overhead for each protection level depends on the file size and the number of nodes in the cluster."