Oracle-L: RE: LGWR using lots of CPU time, low CPU usage

From: John Shaw <John.Shaw_at_correctionscorp.com>
Date: Tue, 26 Nov 2002 12:35:19 -0800
Message-ID: <F001.0050C93A.20021126123519@fatcity.com>

I don't think changing the logmembers will do much good - I have a v880 with 16 GB and a Hitachi san. I have only 3 logmembers - 4 groups, they are on the local drive. I have some kind of performance issue with periodic slowdowns (47 minutes lgwr cpu in one day on a very low transaction system) - still working on the tar. A very odd scenario - an export file which takes 3 minutes to import on my laptop db - takes 8 minutes on to import on the production db if it's on the san. if I move the dmp to the local drive and do the import it takes 20 minutes.

>>> kirti.deshpande_at_verizon.com 11/26/02 01:15PM >>> So what is discussed in this paper is outdated already....uh! www.sun.com/blueprints/0101/SunOracle.pdf Gosh, time flies ;) Pages 13-14 talk about Oracle Redo Logs.

As a first attempt, I would consider reducing the number of log members (from 20 to 4, or even 3) than removing them altogether. This will be of some help right away. But monitor further and decide if more Groups are needed to help archiver process.

Do not change multiple things at the same time.

Good Luck,

Kirti

-----Original Message-----
Sent: Tuesday, November 26, 2002 12:00 PM To: Multiple recipients of list ORACLE-L

We are on 9.2.0.2, Solaris 8 on Sunfire 3800 with 16 GB memory and 128 MB on a hardware-controlled, mirrored RAID5 StorEdge T-3 Array.

Periodically throughout the day the LGWR background process clocks 20+ minutes of CPU time while actual CPU usage is quite low. I ran a statspack report and for a 45-minute period that included the slow LGWR process.

The top 5 timed events in my 45-minute report are:

CPU time 1,295 60.41
db file sequential read 392,516 341 15.91 db file scattered read 70,245 168 7.85
log file sync 26,916 133 6.22
library cache pin 22 59 2.76

(Now that the top 5 is "timed" events, 3 spots almost always include CPU and the db file reads, so I only get two other events, usually log file sync, sometimes enqueue or latch free.)

Statspack also shows the log file parallel write had 28,589 timeouts in that 45 minute period--rather typical for us.

I have session_cached_cursors set to 150.

I am considering the following:

Removing my own redo log duplexing (mirroring) since redo logs are on the mirrored, hardware-controlled RAID5 disk array. (I know, I know) My sysadmin talked to the sun engineer yesterday and he said this is "old school" thinking that redo logs should not be on RAID5. He said because the RAID controller caches to memory all IO requests from the CPUs, all physical writes to disk are done behind the scenes (known as writebehind). He says the system is NOT waiting for IO.
Increasing redo log size (again). For the most part, log switches average 2.5 per day, although there were 20 times in the last month of 3-7 switches in a half hour. My logs are about 100 MB in 2 groups of 20 members each.
Upping the session_cached_cursors to ? (in response to the library cache pin event).

Or is there a better option I'm overlooking?

I would appreciate some advise on the best approach to resolve the slow LGWR process, especially your thoughts on option 1.

Thanks,
Debi
Deborah Lorraine, DBA
University of California, Davis
dlorraine_at_ucdavis.edu

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Author: Deborah Lorraine
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Author: Deshpande, Kirti
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Author: John Shaw
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