EMR scaling is more complex than simply adding or removing nodes from the cluster. One common misconception is that scaling in Amazon EMR works exactly like Amazon EC2 scaling. With EC2 scaling, you can add/remove nodes almost instantly and without worry, but EMR has more complexity to it, especially when scaling a cluster down. This is because important data or jobs could be running on your nodes.
To prevent data loss, Amazon EMR scaling ensures that your node has no running Apache Hadoop tasks or unique data that could be lost before removing your node. It is worth considering this decommissioning delay when resizing your EMR cluster. By understanding and accounting for how this process works, you can avoid issues that have plagued others, such as slow cluster resizes and inefficient automatic scaling policies.
If you’re using EMR today or think you might use it in the future, you should read this.