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In version 5.3, MongoDB introduced a feature called clustered collection which stores documents in the _id index as oppose to the hidden wiredTiger hidden index. This eliminates an entire b+tree seek for reads using the _id index and also removes the additional write to the hidden index speeding both reads and writes. 

However like we know in software engineering, everything has a cost. This feature does come with a few that one must be aware of before using it. In this video I discuss the following 


How Original MongoDB Collections Work
How Clustered Collections Work
Benefits of Clustered Collections
Limitations of Clustered Collections