Humans really cannot fanthom the magnitude of the numbers involved here. 2^64 is an astronomically large number - it&#x27;s mindboggling huge. Really, I will put money on the the claim that YouTube have not seen a collision yet. Let me explain:<p>300 hours every minute, equals 300<i>60</i>24*365.25 = 157.788.000 hours &#x2F; year.<p>Lets assume it&#x27;s all HD and 2 gb an hour, that is ~30pb &#x2F; year. Storing all that video surely uses large capacity harddrive with a 4k sector size, so that it the smallest it makes sense to perform dedupe on.<p>That is ~82726 billion blocks &#x2F; year.<p>To have 50% probability of a collision, we need to calculate 2^64 blocks, so we need 222985 years of video before there is more than 50% probability.<p>Lets move away from YouTube for a moment, and move towards the IP trafic as a whole. Cisco has estimated the total IP trafic to 27483 pb &#x2F; month in 2011, if we&#x27;re looking at the growth it is growing less and less for every year, and from 2010 to 2011 it was only 34% down from 65% from 2005 to 2006. I assume 30% growth from 2011 and forward, and we end up with ~291 exabytes &#x2F; month, or 3497 exabytes &#x2F; year in 2020, this is total IP trafic and will include a lot of duplicated data (e.g. Netflix streams). Again, lets assume a 4kb block, and the total estimated IP trafic in 2020 generates ~961e10^15 blocks. We need 2^64 blocks to have a 50% probability of collision, that is more than 19 times of the total estimated IP trafic in 2020, the vast majority of which is duplicated.<p>My claim is that unless you&#x27;re google, md5 is fine. If you&#x27;re google, md5 is still fine, but you should be looking to prepare your system to accept a hash with a larger output space.