Insane file system overhead on large volume

[Christoph Hellwig]

On Fri, Jan 27, 2012 at 08:50:38AM +0100, Manny wrote:
> Hi there,
> 
> I'm not sure if this is intended behavior, but I was a bit stumped
> when I formatted a 30TB volume (12x3TB minus 2x3TB for parity in RAID
> 6) with XFS and noticed that there were only 22 TB left. I just called
> mkfs.xfs with default parameters - except for swith and sunit which
> match the RAID setup.
> 
> Is it normal that I lost 8TB just for the file system? That's almost
> 30% of the volume. Should I set the block size higher? Or should I
> increase the number of allocation groups? Would that make a
> difference? Whats the preferred method for handling such large
> volumes?

Where did you get the sizes for the raw volume and the filesystem usage
from?