Sharding FAQ

Where do unsharded collections go if sharding is enabled for a database?

In alpha 2 unsharded data goes to the "primary" for the database specified (query config.databases to see details). Future versions will parcel out unsharded collections to different shards (that is, a collection could be on any shard, but will be on only a single shard if unsharded).

When will data be on more than one shard?

MongoDB sharding is range based. So all the objects in a collection get put into a chunk. Only when there is more than 1 chunk is there an option for multiple shards to get data. Right now, the chunk size is 50mb, so you need at least 50mb for a migration to occur.

What happens if I try to update a document on a chunk that is being migrated?

The update will go through immediately on the old shard, and then the change will be replicated to the new shard before ownership transfers.

What if a shard is down or slow and I do a query?

If a shard is down, the query will return an error. If a shard is responding slowly, mongos will wait for it. You won't get partial results.