If memory requirements exceed the available memory of a single server node, a scale out solution consisting of multiple server nodes can be deployed. The architecture of a scale-out solution consists of a master node which stores the row-store data, and several slave nodes which hold partitions of business data tables (InfoCubes, DSOs, PSA tables). By increasing the number of slave nodes, additional storage capacity must be added to the system as needed.
For more detailed information about scale out in general please refer to “SAP Note 1637145 - SAP BW on HANA: Sizing SAP In-Memory Database”. You’ll find there also a report which helps you to estimate the necessary amount of main memory for your BW on HANA scale out landscape.
SAP strongly recommends to have a minimum of three nodes for a HANA scale out database available.