The existing data warehouse cluster remains available for read operations while a new data warehouse cluster gets created during scaling operations. When the new data warehouse cluster is ready, your existing data warehouse cluster will be temporarily unavailable while the canonical name record of the existing data warehouse cluster is flipped to point to the new data warehouse cluster. This period of unavailability typically lasts only a few minutes, and will occur during the maintenance window for your data warehouse cluster, unless you specify that the modification should be applied immediately. Amazon Redshift moves data in parallel from the compute nodes in your existing data warehouse cluster to the compute nodes in your new cluster. This enables your operation to complete as quickly as possible.