According to him, the three important reasons for YouTube's scalability are Python, Memcache and MySQL replication, the last having the most impact. Most people think that the answer to scalability is in upgrading hardware and CPU power. Adding CPUs doesn't work on its own; wisdom is in getting the maximum amount of RAM for the CPU and then fine tuning. He talks about replication in detail sharing his experience in dealing with problems such as time lags in replication between master and slave disks, RAID caching, OS level caching on Linux and cache at the database.
