6.3.6.2 What to Look For When Monitoring PMEM Log

General Performance

Performance issues related to redo logging typically exhibit high latency for the log file sync wait event in the Oracle Database user and foreground processes, with corresponding high latency for log file parallel write in the Oracle Database log writer (LGWR) process. Because of the performance-critical nature of redo log writes, occasional long latencies for log file parallel write may cause fluctuations in database performance, even if the average log file parallel write wait time is acceptable.

If any of these are occurring, then it may be indicative of an issue with PMEM Log performance.

Bypassing PMEM Log

Increased redo write latencies when using PMEM Log can occur when PMEM Log is bypassed. When PMEM Log is bypassed, the request is sent to the cellsrv, and Exadata Smart Flash Log is still used (if available). However, when the bypass request is sent, it has to ensure that there is no conflict with a previous PMEM Log request. This conflict checking, which requires scanning PMEM Log, makes bypass writes more expensive to process, and can result in higher than expected redo log write latencies.

There are several possible causes that result in a small number of PMEM Log bypasses. Under normal circumstances, the number of bypasses should be substantially less than 1% of the total number of PMEM Log requests. A high number of PMEM Log bypasses is likely to be a symptom of another problem, such as congestion on the RoCE Network Fabric.