The processing and storage capacity of a cluster scales linearly as new nodes are added.
In general, you will keep the number of nodes balanced across Cassandra rack configurations when adding nodes. So, if you have three racks in your cluster you will add node three at a time.
Adding new nodes to a cluster can be completed without any interruption to service. However, the process of new nodes synching data with the cluster puts a load on the cluster so we recommend scheduling for a low usage period. The process of a new node joining the cluster make take several hours. The overhead of synching a new node is particular apparent in a small cluster (as the new nodes are a more significant portion of the overall cluster capacity).
It is important to add capacity well before existing capacity (particularly disk space) is exhausted. We do not recommend running any node at >70% disk utilisation in normal operations.
To alter the size of a pre-existing cluster, or increase the number of nodes you can utilise, please contact us using our support services. We expect to introduce self-service for adding nodes to your cluster in a future release.