Frequently Asked Questions: Lock Concept - High availability & Performance

Question

What happens to locks when the enqueue server is restarted?                                                             

Answer

If they have not been saved to disk in the backup file, they will be lost. The locks that are inherited by the update task when COMMIT WORK is executed after CALL FUNCTION .. IN UPDATE TASK are saved to disk. The locks are saved to disk when the update request becomes valid, that is, with the COMMIT WORK. Each time the enqueue server is restarted, the lock entries saved on the disk are reloaded to the lock table.

A lock is saved to disk at the point at which the backup flag is set.     

 

If the enqueue replication service is used as part of a high availability solution, locks will not be lost if the enqueue server fails or is restarted. See next question.

Question

The enqueue server is a single-point-of-failure in the SAP System. Can I guarantee high availability for the Enqueue Server?                   

Answer

To guarantee this you must use the standalone Enqueue Server with the Replication Server.

SAP note 524816 contains the prerequisites that must be fulfilled for using the standalone Enqueue Servers with the Replication Server.

Question

How fast are lock operations?                                

Answer

In work processes on the enqueue server, a few 100 microseconds. In work processes of external application servers you have to include network communications and process changes. Depending on CPU and network load this amounts to a few milliseconds.       

                                    

Question

With a single-process system as an enqueue server, we have reached X  SD Benchmark users. Can this number be increased by using a multiprocessor system (message server on the same machine as the enqueue server)? Can we assume that scaling is linear (number of CPUs * X SD users)? How many processes are advisable if message servers, dispatchers, one dialog, and two enqueue processes are to run on the system?                  

Answer

A significant increase in the enqueue server throughput can be expected by using several processors. The CPU load on the enqueue server is distributed relatively evenly between message servers, dispatchers, and enqueue work processes, which means that up to 3 processors can be occupied simultaneously. The dispatchers and the message server represent the bottleneck with the enqueue.

Linear scaling can be expected for up to three processors, even if lock requests are so frequent that message server, dispatchers, and work processes are occupied simultaneously. Due to asynchronous system processes (for example, syncer), using more processors can further enhance throughput.