Re: DBWR - How Many is Too Many?
Date: Fri, 29 Feb 2008 14:26:01 -0500
Wow! Ask a silly question - get some really good advice! Thanks for all the responses. Found the culprit(s). I did have one controlfile in a CIO FS. But the big hitter was the SAN. When I created the standby on the new hardware in preparation for moving it to the primary, I did so on a SAN on which a firmware upgrade had been performed. The upgrade changed a variety of settings, the most important of which involved a read pre-fetch allocation algorithm. After reading your responses (whilst fending off the forces of management) and doing some additional research, I was able to point the SAN folks in the right direction.
Much obliged to all of you for helping me keep my sanity over the past couple of days. Have a great weekend.
On 2/29/08, goran bogdanovic <goran00_at_gmail.com> wrote:
> I didn't follow the thread to the end, but this note maybe can help you:
> On Wed, Feb 27, 2008 at 7:53 PM, David Barbour <david.barbour1_at_gmail.com>
> > We recently moved our database to a new SAN. Performance has just
> > tanked. Here's the environment:
> > AIX5.3L
> > Oracle 18.104.22.168
> > 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?