Log file sync vs log file parallel write probably not bug 2669566

Question

This is a continuation of a previous thread about ‘log file sync’ and ‘log file parallel write’ events.
Version : 9.2.0.8
Platform : Solaris
Application : Oracle Apps
The number of commits per second ranges between 10 and 30.
When querying statspack performance data the calculated average wait time on the event ‘log file sync’ is on average 10 times the wait time for the ‘log file parallel write’ event.
Below just 2 samples where the ratio is even about 20.
"snap_time"     " log file parallel write avg"     "log file sync avg"     "ratio
11/05/2008 10:38:26      8,142     156,343     19.20
11/05/2008 10:08:23     8,434     201,915     23.94
So the wait time for a ‘log file sync’ is 10 times the wait time for a ‘log file parallel write’.
First I thought that I was hitting bug 2669566.
But then Jonathan Lewis is blog pointed me to Tanel Poder’s snapper tool.
And I think that it proves that I am NOT hitting this bug.
Below is a sample of the output for the log writer.
-- End of snap 3
HEAD,SID, SNAPSHOT START ,SECONDS,TYPE,STATISTIC , DELTA, DELTA/SEC, HDELTA, HDELTA/SEC
DATA, 4, 20081105 10:35:41, 30, STAT, messages sent , 1712, 57, 1.71k, 57.07
DATA, 4, 20081105 10:35:41, 30, STAT, messages received , 866, 29, 866, 28.87
DATA, 4, 20081105 10:35:41, 30, STAT, background timeouts , 10, 0, 10, .33
DATA, 4, 20081105 10:35:41, 30, STAT, redo wastage , 212820, 7094, 212.82k, 7.09k
DATA, 4, 20081105 10:35:41, 30, STAT, redo writer latching time , 2, 0, 2, .07
DATA, 4, 20081105 10:35:41, 30, STAT, redo writes , 867, 29, 867, 28.9
DATA, 4, 20081105 10:35:41, 30, STAT, redo blocks written , 33805, 1127, 33.81k, 1.13k
DATA, 4, 20081105 10:35:41, 30, STAT, redo write time , 652, 22, 652, 21.73
DATA, 4, 20081105 10:35:41, 30, WAIT, rdbms ipc message ,23431084, 781036, 23.43s, 781.04ms
DATA, 4, 20081105 10:35:41, 30, WAIT, log file parallel write , 6312957, 210432, 6.31s, 210.43ms
DATA, 4, 20081105 10:35:41, 30, WAIT, LGWR wait for redo copy , 18749, 625, 18.75ms, 624.97us
When adding the DELTA/SEC (which is in micro seconds) for the wait events it always roughly adds up to a million micro seconds.
In the example above 781036 + 210432 = 991468 micro seconds.
This is the case for all the snaps taken by snapper.
So I think that the wait time for the ‘log file parallel write time’ must be more or less correct.
So I still have the question “Why is the ‘log file sync’ about 10 times the time of the ‘log file parallel write’?”
Any clues?

Answer 1

Yes that is true!
But that is the way I calculate the average wait time = total wait time / total waits
So the average wait time for the event 'log file sync' per wait should be near the wait time for the 'llog file parallel write' event.
I use the query below:
select snap_id
, snap_time
, event
, time_waited_micro
, (time_waited_micro - p_time_waited_micro)/((snap_time - p_snap_time) * 24) corrected_wait_time_h
, total_waits
, (total_waits - p_total_waits)/((snap_time - p_snap_time) * 24) corrected_waits_h
, trunc(((time_waited_micro - p_time_waited_micro)/((snap_time - p_snap_time) * 24))/((total_waits - p_total_waits)/((snap_time - p_snap_time) * 24))) average
from (
select sn.snap_id, sn.snap_time, se.event, se.time_waited_micro, se.total_waits,
lag(sn.snap_id) over (partition by se.event order by sn.snap_id) p_snap_id,
lag(sn.snap_time) over (partition by se.event order by sn.snap_time) p_snap_time,
lag(se.time_waited_micro) over (partition by se.event order by sn.snap_id) p_time_waited_micro,
lag(se.total_waits) over (partition by se.event order by sn.snap_id) p_total_waits,
row_number() over (partition by event order by sn.snap_id) r
from perfstat.stats$system_event se, perfstat.stats$snapshot sn
where se.SNAP_ID = sn.SNAP_ID
and se.EVENT = 'log file sync'
order by snap_id, event
where time_waited_micro - p_time_waited_micro > 0
order by snap_id desc;