Re: DBWR - How Many is Too Many?

Hi,

I didn't follow the thread to the end, but this note maybe can help you:

*316533.1

HTH,
goran

*
On Wed, Feb 27, 2008 at 7:53 PM, David Barbour <david.barbour1_at_gmail.com> wrote:

> 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?
>
>

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