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Another gotcha with sharding is that during a re-balance some documents will be on "both" shards simultaneously. Thus issuing a count command repeatedly on a 100k doc collection will see the count bounce up as it shards. 100k, 106k, 100k, 104k etc. This is not a problem for lookups of single documents but means any row scans will produce inconsistent values if the collection is re-balancing. There is a certain mode you can trigger for scan queries that may address this but I have not tested it yet.

You learn a lot about mongodb just by playing with it. It is super simple to set up and just hammer with different tests. I'm playing around with 4 extra large instances wired to 4 drives each in raid 0 on amazon right now and having a blast.

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