Because SAP ERP is one of the most important transactional systems in a typical enterprise IT environment, the system architecture must consider both performance and availability. Dell EMC highly recommends a distributed system architecture, where each of the main components resides on its own VM, as follows:
- ABAP central services (ASCS) instance―ASCS comprises a message server and an enqueuer server that are both SPOFs. Separating ASCS from an application server instance in a central system architecture can minimize the impact of other work processes. A lower chance of failure also provides for the highest level of protection by using VMware fault tolerance. The SAP shared file systems /sapmnt/<SID> and /usr/sap/<SID> can be stored on this instance and shared to all other SAP instances within the same system.
- Database instance―A dedicated database instance has full command of its VM resources and is isolated from any other possible threat to the stability of the database. Because the network traffic between the database instance and the application server instances is usually high and RAM state change within the VM is frequent, avoid using VMware FT to protect the database instance. Use operating system/database-specific clustering tools or vSphere HA to provide a higher level of protection, but do not use both together.
- Additional application server (AAS) instances―AAS is a scale-out architecture that performs most of the computational tasks when transactions and background jobs are executed. You can add AAS at any time for additional performance and availability. Login groups (T-Code: SMLG) usually manage access to provide flexibility and increase availability. If one application server instance fails, connected users lose connection and reconnect to other AAS instances, and transaction-in-flight is rolled back. Standard vSphere HA is sufficient to provide a quick restart from ESXi or OS failures.