The “Transaction Delay” value is an accumulation of the delay of all the current transaction delay in millisecond. You can see that the “Transaction Delay” counter has the same spikes as the sudden drop of the “Transactions Created/Sec”. Its spikes indicate that at those time points the AG transactions have time delay during commits. This gives us a very good start point. We can focus on the transaction delay in our AG performance troubleshooting.
So who causes the transaction delay? Is it primary replica, secondary replica, or other factors like network traffic?
As a must go-through step for performance troubleshooting we captured performance monitor logs to check how the performance behaved on both replicas. We want to find out whether there is any performance bottleneck existing in primary or secondary. For example, whether CPU usage is high when transaction delay spike happens, whether disk queue length is long, disk latency is large, etc. We expect to find something that has the same spike trend as the “Transaction Created/sec” or “Transaction Delay”. Unfortunately, we do not anything interesting. CPU usage is as low 30%, Disk speed is quite fast. No disk queue length at all. We then checked AG related counters, like the log send queue and the recovery queue as the above two links mentioned but again we do not find anything helpful.
At the endpoint, there’s a reminder that you should keep up to date on patching systems.