eG Monitoring
 

Measures reported by OraRacComModTest

A wait class is a grouping of wait events, and every wait event belongs to a wait class. The main wait classes of the Oracle database server are:

  • Administrative
  • Application
  • Cluster
  • Commit
  • Concurrency
  • idle
  • Network
  • Other
  • Scheduler
  • System I/O
  • User I/O
The Commit wait class comprises of only one wait event - wait for redo log write confirmation after a commit (that is, 'log file sync'). Commit is not complete until LGWR (log writer) writes log buffers including commit redo records to log files. In a nutshell, after posting LGWR to write, user or background processes waits for LGWR to signal back with 1 sec timeout. User process charges this wait time as 'log file sync' event.

This test reports the number of sessions to each instance, which are waiting for a redo log write confirmation after a commit. This way, the test sheds light on the open sessions to a instance, and the reason for the sessions remaining open.

The measures made by this test are as follows:

Measurement Description Measurement Unit Interpretation
Number_of_session Indicates the number of sessions to this instance that are waiting for redo log write confirmation after a commit. Number A steady increase in the value of this measure is a cause of concern, as it indicates the following:
  • Many sessions are forced to stay open owing to the commit wait events, and this may cause a session overload;
  • Many "log file sync" wait events are occuring, causing the performance of the Oracle RAC to deteriorate. The root cause for 'log file sync' waits are as follows:
    • LGWR is unable to complete writes fast enough - this could be because, the disk I/O performance to log files is not good enough or, the LGWR is starving for CPU resources or, the LGWR paged out due to memory starvation issues or, due to file system or unix buffer cache limitations

    • LGWR is unable to post the processes fast enough, due to excessive commits.

    • IMU undo/redo threads

    • LGWR is suffering from other database contention such as enqueue waits or latch contention

    • Various bugs

Use the detailed diagnosis of this measure to view the details of the sessions affected by log file sync waits.