Re: DBWR - How Many is Too Many?
- From: "David Barbour" <david.barbour1@xxxxxxxxx>
- To: "Greg Rahn" <greg@xxxxxxxxxxxxxxxxxx>
- Date: Wed, 27 Feb 2008 17:27:12 -0500
Good point(s). And 'free buffer waits' is NOT in my top 5.
On 2/27/08, Greg Rahn <greg@xxxxxxxxxxxxxxxxxx> wrote:
>
> My first question to you (and others who increase the number of dbwr
> processes): Are you seeing 'free buffer waits' in your 'Top 5' in the
> Statspack report? If not, it is unlikely that adding more dbwr process will
> yield any benefit.
>
> If you are experiencing high wait IO (>40% is high), then what benefit is
> there to potentially do more IO, by adding dbwrs? I would think that
> would make things worse. You need to get to the root cause of why the WIO
> is high. My suggestion is start by investigating the iostat data (what are
> the IO response times, lun queue depth, etc). It sounds quite likely that
> if you even use a simple file create test, the IO would be poor. I would
> use /usr/sbin/lmktemp and do some lower level testing.
>
> The other observation I would make is that if you changed the SAN, and not
> the database, and it worked ok before, then in all likelihood it is not a
> database problem. No?
>
>
> On 2/27/08, David Barbour <david.barbour1@xxxxxxxxx> wrote:
> >
> > sar is scary (just a small portion)
> >
> > 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).
>
>
> > 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?
> >
> >
>
>
> --
> Regards,
>
> Greg Rahn
> http://structureddata.org
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