Re: DBWR - How Many is Too Many?

David Barbour wrote,on my timestamp of 28/02/2008 5:53 AM:
We recently moved our database to a new SAN. Performance has just tanked. Here's the environment: 
AIX5.3L
Oracle 9.2.0.7 <http://9.2.0.7>
SAN - IBM DS4800



Aye...


sar is scary (just a small portion)

AIX r3prdci1 3 5 00CE0B8A4C00    02/27/08

System configuration: lcpu=8

00:00:00    %usr    %sys    %wio   %idle   physc
02:15:01      19      19      42      19    4.00
02:20:00      21      25      40      14    4.00
02:25:00      19      18      43      20    4.00
02:30:00      18      18      43      21    4.00
02:35:00      20      24      40      16    4.00



wio is w-a-a-a-a-a-y too high with sys time low,
might well be a I/O misconfiguration.



1.  One db writer for each database disk - in our case that would be 8
2. CPUs/8 adjusted for multiples of CPU groups - in our case that would be 4 

Any thoughts?


Large datablock waits in db, plus lots of wio
in the OS.

I'm thinking possible misconfiguration
of the AIX Disk I/O pacing parameters.
From memory, it's:
smitty chgsys
and check all file systems for minpout and maxpout
values.  Unless you know in detail the I/O balance
of your system, you're much better off with these
two set at 0 (zero).

There are papers at IBM on this subject, gogle
for "IBM AIX disk I/O Pacing".

Had a similar situation a while ago: a new SAN's
file systems for some reason ended up with 30/30
for both pout pars.  When we set them to 0, the
wio dropped very significantly.

At least worth checking.

--
Cheers
Nuno Souto
in rainy Sydney, Australia
dbvision@xxxxxxxxxxxx
--
http://www.freelists.org/webpage/oracle-l

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