Raidz in FreeNAS eating more space than expected

I just got 6 new 2TB drives, and added them to my FreeNAS box. I have only dealt with RAID1 previously, and each setup has given what I was expecting.

However, with the 6*2TB drives, I wanted to maximize the space available, so I went with raidz. But I seem to be missing space. I have 8.6TB available after the raidz was built. Maybe I did my math horribly wrong, but (N-1) x S(min) (where N=6 and S(min)=2TB) should result in 10TB. (I understand it would be more like 9.something)

Does raidz actually consume more then 1 drive worth of space? Or could their possibly be another problem? (All drives have been independently verified that 2TB of space is available)

Solution 1:

Freenas/Zfs reserves a small fragment of drive space. So besides having only ~1.82TB of actual space. ZFS reserves 1/64th of drive space for its own means, thus 'stealing' another ~28gb from you on every drive. Also freenas makes a 2gb swap file on every drive, Then losing the 1 drive to Raidz, 8.6TB seems pretty close.

Source: http://cuddletech.com/blog/?p=261

edit Freenas swap file on every drive added.