Parity takes 1/9th of all space (we write 9 physical sectors of ECC'd data for each 8 logical sectors of data, which allows realtime correction of single and double bit flip errors even if duplication is not enabled), hence 75 GB --> 66 GB. We also
pre-allocate 3 GBs per volume/share for MFT and other NTFS metadata files (those 3 GBs are not fully used though, so user data can be written into them if a need arises), so you see 18 GB allocated since we have six default shares. The 3 GB is a fixed number,
so the relative impact of those will be greatly reduced if you use large drives (and we expect the majority of Vail servers to ship with 1TB+ drives).
Bulat Shelepov, Test Lead (Drive Extender), Windows Home and Small Business Server Team