Hi Eric,

I re-read "TSM implementation guide" Redbook a couple of minutes ago,
and they don't speek about any overhead space for a data  volume, what
they specify is that if you want to format a db volume, then the
allocated file size is always a multiple of 4 MB plus an additional 1 MB
for overhead.
Something else comes to my mind : when preparing your VG, did'nt you
forgot  the "large file enable" parameter ?
Hope this helped, but probably somebody has a better knowledge than I
have ....
Cheers.
Arnaud