Clarification on scaling mongodb

Let's say I start with replica sets and each mongo server has a 500G hard disk attached to each one. What happens if my data grows beyond 500G? I assume that's where sharding comes into play? In addition, I keep reading that in order to gain optimal performance, you would want to keep your indexes in memory. So if my index is 100G, does that mean EAC开发者_JAVA百科H server (shard or non-shard) has to have at least 100G of RAM?

If you data grows beyond the size of your disk, yes, that's a good time to start sharding. MongoDB will start reporting errors that it couldn't do inserts/updates that increase size once you run out of disk space, so you should shard a before you get to that point.

You really want to be able to keep the working set of your indexes (and data, if possible) in memory. So, if your doing a website and mostly people access the last week of content, if you have enough RAM to keep the last week in memory, you should be fine (even if you have a couple years of content on disk).