sadly if fossil overflows it is terminal. having said this you can refresh your fossil from the last snapshot, so not everything has gone, but the rule is don't let fossil overflow.
your venti size sounds very big. Perhaps you have a lot of media files, but remember that duplicate files are merged and take up only one file's worth of space (de-duplication is done on the files contents not on its name. also all data is compressed so it takes less space. -Steve
