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Oracle wait event ‘log file parallel write’ change

This post is about a change in how the time is measured for the event ‘log file parallel write’. This is important for the performance tuning of any change activity in an Oracle database, because with the default commit settings, a foreground session that commits changes waits in the wait event ‘log file sync’, which is a wait on logwriter activity, for which the wait event ‘log file parallel write’ always has been the indicator of the time spend on IO.

Log file sync
First things first: a foreground session normally waits on the wait event ‘log file sync’ when it commits waiting for its change vectors to be written to the online redologfile(s) by the logwriter. It is wrong to always assume a ‘log file sync’ will be present. If, somehow, the logwriter manages to increase the ON DISK SCN to or beyond the foreground session’s commit SCN, there will be no ‘log file sync’ wait event.