One last thought
You don't want to compress archived data, because that increases the vulnerability of the data to damage. Not only can one cosmic ray zap more (virtual) bits, but it can have cascading consequences in streaming compression formats. That is, if you knock out a block of data in the middle, the decompressor can lose sync and be unable to recover everything that comes after. We saw this a lot in MGS images, where large blocks came out black as a result of losing only a few bits in the middle. With all the storage capacity we have now and coming in the future, it's penny-wise pound-foolish to compress archival data.