How does elastic resize differ from classic resize?

Both Elastic resize and classic resize are used to scale the Redshift cluster up or down. With classic resize, a new cluster is created with the required nodes, and data is copied from the existing cluster to the new one. Once it is completely copied, the existing cluster is terminated, and the new cluster is renamed to the existing one. Throughout the course of classic resize, the cluster is available in the read-only mode.

In elastic resize, new nodes are added/removed from the existing cluster within minutes. Data is distributed in the background. The cluster is available for read and write during data distribution.