does ‘redo write time’ include “log file parallel write”
649748Apr 15 2010 — edited Apr 15 2010IO performance Guru Christian said in his blog:
http://christianbilien.wordpress.com/2008/02/12/the-%E2%80%9Clog-file-sync%E2%80%9D-wait-event-is-not-always-spent-waiting-for-an-io/
The log file sync wait can be broken into:
1. The ‘redo write time’: this is the total elapsed time of the write from the redo log buffer to the current redo log file (in centiseconds).
2. The “log file parallel write” is basically the time for the log write I/O to complete
3. The LGWR may have some post processing to do, then signals the waiting foreground process that the write has completed. The foreground process is finally waken up by the system dispatcher. This completes the ‘log file sync’ wait.
So in his idea, there is no overlap between ‘redo write time’ and “log file parallel write”
But in the metalink 34592.1:
The log file sync wait may be broken down into the following components:
1. Wakeup LGWR if idle
2. LGWR gathers the redo to be written and issue the I/O
3. Time for the log write I/O to complete
4. LGWR I/O post processing
...
Tuning advice based on log file sync component breakdown above:
Steps 2 and 3 are accumulated in the "redo write time" statistic. (i.e. as found under STATISICS section of Statspack and AWR)
Step 3 is the "log file parallel write" wait event. (Note.34583.1:"log file parallel write" Reference Note:)
So metalink said there is overlap as "redo write time" include both step 2 and step, and "log file parallel write" only include step 3, so time of "log file parallel write" is only part of time of "redo write time". Is the metalink note wrong, or I missed something?