I know this will sound like bashing MongoDB. But I’ve already said it a couple of times: MongoDB scaling looks complicated.
People coming to MongoDB think their scalability issues are of no concern as MongoDB supports auto-sharding and replica sets. I would advise everyone to check the MongoDB mailing list before relaxing:
- “Huge collection is not balanced across shards” here
- “Questions on Enabling Sharding on a Database (pre-sharded configuration)” here
- “mongodb crash when new shard is added” here
- “Shards Unevenly balanced” here
- “Imbalance on auto-sharding” here
- “Sharding woes - All chuncks stuck on one shard”
- “sharding makes reads/writes slower” here
It takes time to perfect complicated solutions and it is your job to verify the current status. Hopefully the next version, MongoDB 1.8.0 will address many of these issues. As it’ll also do with single server durability.