9.1.16 New Metrics for Redo Log Writes

New metrics are available to help analyze redo log write performance.

Previously, when Automatic Workload Repository (AWR) reported an issue with redo log write wait time for database servers, the storage cells often indicated no issue with redo log write performance. New metrics help to give a better overall picture. These metrics provide insight into the following concerns:

• Is the I/O latency high, or is it some other factor (for example, network) ?

• How many redo log writes bypassed Flash Log ?

• What is the overall latency of redo log writes on each cell, taking into account all redo log writes, not just those which were handled by Flash Log ?

Oracle Exadata System Software release 12.2.1.1.0 introduces the following metrics related to redo log write requests:

• FL_IO_TM_W: Cumulative redo log write latency. It includes latency for requests not handled by Exadata Smart Flash Log.

• FL_IO_TM_W_RQ: Average redo log write latency. It includes write I/O latency only.

• FL_RQ_TM_W: Cumulative redo log write request latency. It includes networking and other overhead.

  To get the latency overhead due to factors such as network and processing, you can use (FL_RQ_TM_W - FL_IO_TM_W).

• FL_RQ_TM_W_RQ: Average redo log write request latency.

• FL_RQ_W: Total number of redo log write requests. It includes requests not handled by Exadata Smart Flash Log.

  To get the number of redo log write requests not handled by Exadata Smart Flash Log, you can use (FL_RQ_W - FL_IO_W).