How will sharded cluster take to initialize?

[Travis Beauvais]

I'm currently moving a replica set to a sharded cluster. When I first enable sharding on the collection, will it take a long time to initialize? The collection is about 700gb. I just want to know what kind of downtime I should expect to have. In my tests with a small cluster it was instant but that was with significantly less data.

[Travis Beauvais]

Maybe a question is when is it safe to start accessing the cluster through the mongos?

I just attempted to deploy the sharded cluster. I added the shard and the balancer started working. Only a single shard so I'd hope there wouldn't be much but I guess it has to build an index? I did some small tests and even though the balancer was still running I could access data through the mongos so I turned on my clients. The clients worked fine for about 10 minutes and then the mongos seemed to have just froze. Even after I stopped the clients mongos was still showing more than 20k open connections (which shouldn't have been this high to begin with).

Do I need to let the balancer completely index the data before I start accessing it through the mongos?

At what point is it no longer recommended to access through the mongod?