RE: ** log_buffer
- From: A Joshi <ajoshi977@xxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx, ajoshi97@xxxxxxxxx, mwf@xxxxxxxx
- Date: Wed, 23 Jul 2008 14:55:41 -0700 (PDT)
Mark,
Thanks a lot for the explanation. My main problem is log file sync. I have
very high log file sync waits (highest wait event) and it waits almost half a
second for each which I would like to reduce and I also have high log buffer
wait and it is not as high as log file sync but in the top five.
Thanks a lot.
--- On Tue, 7/22/08, Mark W. Farnham <mwf@xxxxxxxx> wrote:
From: Mark W. Farnham <mwf@xxxxxxxx>
Subject: RE: ** log_buffer
To: ajoshi977@xxxxxxxxx, oracle-l@xxxxxxxxxxxxx, ajoshi97@xxxxxxxxx
Date: Tuesday, July 22, 2008, 1:37 PM
If you’re driving log file syncs as fast as you can at a steady state of load
and you can’t keep up, then increasing log buffer won’t have a material effect
on your total throughput. In that case if you need greater throughput you need
to find a way to either do less log file syncs per unit work or a way to drain
them faster.
If your load (in this case of things that drive log file syncs) is peaked, then
increasing log buffer has the same characteristics of pushing off when you’ll
hit log buffer space waits as any other cache. Just because something starts
flushing for any given trigger doesn’t fundamentally mean more can’t be used in
parallel with the flushing. That is why these things are designed as “circular”
buffers.
Finding the exact optimal size for log_buffer is usually a fool’s errand
compared to the cost and availability of memory on today’s servers. Setting it
a little bigger than you think you need is usually cost effective. Do remember
that you can’t cut out more time from your total throughput than the time of
the log buffer space waits plus the side effect costs of the context switches
those waits cause. Log buffer space waits tend to introduce significant
avoidable costs only for batch jobs with large infrequent commits since most
other jobs will frequently yield for a commit or not need log buffer space at
all.
I hope this helps.
mwf
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On
Behalf Of A Joshi
Sent: Tuesday, July 22, 2008 12:01 PM
To: oracle-l@xxxxxxxxxxxxx; ajoshi97@xxxxxxxxx
Subject: ** log_buffer
Hi,
Is any specific formula to find the optimum log_buffer size. Any
considerations. I read that any size above 3MB might not help since it flushes
at that point anyway. Database version 9i. We are seeing high log file sync
waits and also log buffer space waits. Thanks
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