log file sync wait

The log file sync event occurs when a user issues Commit or Rollback. Click here for the difference between log file sync and log parallel write wait event.

When a user issues a commit or rollback command, the redo data in the redo buffer is written to online redo log file. This write is known as sync write. During this synchronization process the user process waits in log file sync event, while the LGWr waits on log file parallel write event.

The log file sync event is very fast and usually unnoticed by the end users. However you may notice that there are very high time waited for this wait event in certain cases. The main cause for such high wait for this event is as follows,

Too many commits

If you notice high waits at session level then it may be due to running batch processes there are commits within a loop. If that is the case then the application logic can be changed by eliminating unneccessary commits and reduce commit frequency.

If you notice high waits at system level then it may be due to short transactions. OLTP databases usually have short transactions and have high log file sync wait events. Only thing you can do to improve the performance, in this case, is to use faster IO subsystem, rawdevices.

Large Log buffer

The redo entries from buffer to log files take place either through sync writes as explained earlier or through background writes (Such as 1/3 full, 1MB redo etc). When redo log buffer is large then more redo data are accumulated in the buffer. The background writes (i.e., when redo becomes 1/3 full ) are limited or delayed. When a user issues a commit or roll back then the sync writes will take more time.

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