Re: DBWR - How Many is Too Many?

David,

I would suggest that you check SAP documentation and Oracle's AIX specific documents for recommendations on db_writers and async_io before making changes. Several things came to mind after reading your post and Paul's question:

1. I used multiple db writers for our SAP system due to issues with async i/o on HP-UX. You don't have these same issues on AIX and thus, multiple db_writers may not be your best answer.
2. Our SAP database is one third the size of the database you've described (we archive a lot of data with SAP archive tools). We used mirrored drives for all data files, and data files were spread over about 65 data file mount points. Writes on RAID-5 will be your biggest issue and SAP is write intensive.
3. Still suggest that you gather system stats to get some numbers on read and cpu speeds.

Here's a link to Oracle's recommendations for 9i on AIX:

http://download.oracle.com/docs/html/A90347_02/appa_aix.htm#631246

Robyn

On Wed, Feb 27, 2008 at 2:24 PM, David Barbour <david.barbour1_at_gmail.com> wrote:

> Yes. Identical SAN - IBM DS4800. Only 4 Oracle datafile filesystems.
> Size of the database (5.4TB) necessitated we move it to accomodate
> growth. Split index and data files, took undo and temp and put them in
> seperate filesytems too. I actually expected performance to improve.
>
> On 2/27/08, Baumgartel, Paul <paul.baumgartel_at_credit-suisse.com> wrote:
> >
> > 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.
> >
> > 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?
> >
> > ==============================================================================
> > Please access the attached hyperlink for an important electronic communications disclaimer:
> >
> > http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html
> >
> > ==============================================================================
> >
> >
>

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