MongoDb - Utilizing multi CPU server for a write heavy application

So right now, the easy solution is to shard.

Yes, normally sharding is done across servers. However, it is completely possible to shard on a single box. You simply fire up the shards on different ports and provide them with different folders. Here's a sample configuration of 2 shards on one box.

The MongoDB team recognizes that this is kind of sub-par, and I know from talking to them that they're looking at better ways to do this.

Obviously once you get multiple shards on one box and increase your write threads, you will have to be wary of disk IO. In my experience, I've been able to saturate disks with a single write thread. If your inserts/updates are relatively simple, you may find that extra write threads don't do anything. (Map-Reduces are the exception here, sharding definitely helps there)

No, it is still recommended to use sharding to utilize multiple CPU cores. As stated in the FAQ

Sharding improves concurrency by distributing collections over multiple mongod instances, allowing shard servers (i.e. mongos processes) to perform any number of operations concurrently to the various downstream mongod instances.

Each mongod instance is independent of the others in the shard cluster and uses the MongoDB readers-writer lock). The operations on one mongod instance do not block the operations on any others.

Sharding on a single box has its issues, as one user stated in the mongodb-user mailing list

After some significant experimentation, I've found a single MongoDB shard daemon CANNOT use more than one CPU. On a 24 CPU box, performance scales up until we hit about 8 shards, then another limit kicks in.