Redo-related wait events

There are a number of wait events that happen during redo activities and most of them are I/O related. First, I will talk about the two most important wait events: 'log file parallel write' and 'log file sync'. Then I will mention some other important ones you should know about.

The 'log file parallel write' event

Oracle foreground processes wait for 'log file sync', whereas the LGWR process waits for 'log file parallel write'. Although we usually find 'log file sync' in the Top 5 Timed Events or the Wait Events section of the Statspack report, in order to understand it we will first look at 'log file parallel write'.

The LGWR background process waits for this event while it is copying redo records from the memory log buffer cache to the current redo group's member log files on disk. Asynchronous I/O will be used if available to make the write parallel, otherwise these writes will be done sequentially one member after the other. However, LGWR has to wait until the I...