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If a shard say has 200 chunks on it and it is time to move some of those chunks to another shard,
1>how does mongo db decide which chunks to move?
2>Is this move logic in config server or mongos?
3>How can I affect/control chunk selection algorithm such that mongodb will move chunks to other shards such that it helps to distribute my reads based on my users access pattern?
Movement of chunks between shards is triggered by mongos. Mongos will move chunks under two circumstances. If one shard contains 9 or more chunks than any other, mongos will trigger a balancing round and redistribute the chunks between the other shards. In this situation, the chunks with the lowest shard keys will be moved. Additionally, if the top most chunk is split, mongos will move the chunk with the higher shard key to another shard.
One of the features of Mongo is that in a properly set-up sharded cluster, chunks are split and moved automatically such that your application does not need to be aware that it is interacting with a sharded database. Everything happens behind-the-scenes.
However, it is possible to split and move chunks manually using the “split” and “moveChunks” commands. Please see the mongo documentation for examples of how to use these commands: “Splitting Shard Chunks” http://www.mongodb.org/display/DOCS/Splitting+Shard+Chunks and “Moving
Chunks” http://www.mongodb.org/display/DOCS/Moving+Chunks There have been cases where users have written their own custom balancers taylored to their own applications, but this is not common, and only attempted by the most advanced of users.
As an alternative, it is possible to give the balancer window of time when it may operate, or to disable it entirely. Some users will temporarily disable the balancer for maintenance, or give it a window so it is not competing for write locks at times when they expect their application to be putting the db under high loads.
More information on the balancer is available in the “Balancing” and “Balancer window” sections of the “Sharding Administration” documentation.
http://www.mongodb.org/display/DOCS/Sharding+Administration
Hopefully the above resources will give you a better understanding of how sharding works, and how chunks are balanced between shards.