You'll have to evaluate how much history you need. I don't think you have to save the whole history of the blockchain. You could make something that purged the database of outdated history. That way your database would not get to big.

If you set a hard timelimit on the comments that would be counted like maybe only comments within 7 or 14 days.
This would reduce the amount of data you would have to keep stored at any given time.