The US equity market does about 4b in transactions a day; let's say the average trade size is $1000 dollars (probably an aggressive estimate; I'd expect it actu

The US equity market does about 4b in transactions a day; let's say the average trade size is $1000 dollars (probably an aggressive estimate; I'd expect it actually to be higher), and you record the ticker (4bytes) + price(8) + quantity(8) for all the transactions, you'd get -- (4+8+8)(4e9/1000) 252 (trading days) / 1e9 = 20GB/year lets say you have these for 100 years, you'd still only have 2TB. And this if for the entire US equity market -- NYSE is only one of 11 protected exchanges (and only trades tape A), so the above estimates are pretty optimistic. Now, if you wanted to record every message and not just every transaction, then you get into serious data (on the order of ~2-5TB/day).