RE: DBWR - How Many is Too Many?
Date: Wed, 27 Feb 2008 14:03:06 -0500
Message-ID: <21469B88E0EA11498818517F2103353101C65FB6@EPRI17P32001A.csfb.cs-group.com>
One question: were you on RAID 5 previously?
Paul Baumgartel
CREDIT SUISSE
Information Technology
Prime Services Databases Americas
One Madison Avenue
New York, NY 10010
USA
Phone 212.538.1143
paul.baumgartel_at_credit-suisse.com
www.credit-suisse.com
From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org] On Behalf Of David Barbour
Sent: Wednesday, February 27, 2008 1:53 PM
To: oracle-l_at_freelists.org
Subject: DBWR - How Many is Too Many?
We recently moved our database to a new SAN. Performance has just
tanked. Here's the environment:
AIX5.3L
Oracle 9.2.0.7
SAN - IBM DS4800
We've got 8 filesystems for Oracle data files. Redo, Archive, Undo and
Temp are all on seperate disk/filesystems from the data files.
All the Oracle datafiles are on RAID5 LUNs with 12 15K RPM 73 (68 usable) GB drives. SAN Read and Write Caching are both enabled.
A statspack (generally for any given interval - this was for a period of
"light" processing) shows me our biggest hit is:
Buffer wait Statistics for DB: PR1 Instance: PR1 Snaps: 12609 -12615
-> ordered by wait time desc, waits desc
Tot Wait Avg
Class Waits Time (s) Time (ms)
------------------ ----------- ---------- ---------
data block 278,194 20,811 75
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
We're running JFS2 filesystems with CIO enabled, 128k element size on the SAN and AIO Servers are set at minservers = 220 and maxservers = 440 We've got 32GB of RAM on the server and 4 CPUs (which are dual core for all intents and purposes - they show up as eight). We're running SAP which has it's own memory requirements. I've configured my SGA and PGA using Automatic Memory Management and the SGA currently looks like: SQL> show sga
Total System Global Area 1.0739E+10 bytes
Fixed Size 757152 bytes Variable Size 8589934592 bytes Database Buffers 2147483648 bytes Redo Buffers 1323008 bytes
filesystemio_options = setall
I'm thinking the data block waits is the result of too many modified blocks in the buffer cache. Solution would be to increase the number of db_writer_processes, but we've already got 4. Metalink, manuals, training guides, Google, etc. seem to suggest two answers.
- One db writer for each database disk - in our case that would be 8
- CPUs/8 adjusted for multiples of CPU groups - in our case that would be 4
Any thoughts?
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