Very important, not entirely unproblematic, suggest reading:

Blog post found in response to somebody's question on Quora "Why aren't Flickr's photo IDs sequential":" Sharding (aka data partioning) is how we scale Flickr’s datastore. Instead of storing all our data on one really big database, we have lots of databases, each with some of the data, and spread the load between them. Sometimes we need to migrate data between databases, so we need our primary keys to be globally unique. Additionally our MySQL shards are built as master-master replicant pairs for resiliency. This means we need to be able to guarantee uniqueness within a shard in order to avoid key collisions. We’d love to go on using MySQL auto-incrementing columns for primary keys like everyone else, but MySQL can’t guarantee uniqueness across physical and logical databases."