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Re: os cache vs. db cache

From: Niall Litchfield <niall.litchfield_at_gmail.com>
Date: Tue, 10 Jul 2007 14:42:12 +0100
Message-ID: <7765c8970707100642h46513b82o75edc24e9ef1ab50@mail.gmail.com>


Nice observation Chris, and I suspect quite common, especially if you also include sites where the hardware cache does the same job. Also 'accidentally'.

On 7/10/07, Chris Dunscombe <chris_at_thedunscombes.f2s.com> wrote:
>
> Hi,
>
> One situation I've experienced was a smallish (< 250GB) third-party online
> operational database on Solaris where the OS cache acted as a cache for
> Full
> Table scans of tables around the 100-300 MB size. This worked well
> although it
> was more by accident than design.
>
> Cheers,
>
> Chris
>
>
> Quoting Robyn <robyn.sands_at_gmail.com>:
>
> > Thank you Mark and Brandon,
> >
> > This is the kind of information I'm looking for; I've read Steve's stuff
> but
> > it's been a while and the AIX paper is new to me. I *think* we need to
> make
> > some changes in our approach, but right now, I just want to gather and
> study
> > as much information on the different options and approaches as possible.
> >
> > So, if anyone has additional links, documents or experiences, I'd
> appreciate
> > the input.
> >
> > thank you ... Robyn
> >
> > On 7/9/07, Allen, Brandon < Brandon.Allen_at_oneneck.com> wrote:
> >>
> >> I'd be curious to hear anyone's reasons for preferring OS cache to DB
> >> cache.
> >>
> >> It seems pretty clear cut to me that it is better to allow Oracle to
> >> manage its own cache since it has much more knowledge available
> internally
> >> to help predict which blocks are most likely to be needed again. I've
> had
> >> good results with CIO (Concurrent, a.k.a non-buffered,
> non-inode-locking
> >> I/O) on AIX, but I did increase db_cache_size to make up for the lack
> of
> >> filesystem buffering - in one case from 600M to 1500M, in another I
> just
> >> used CIO from the beginning so there was no before/after comparison,
> but
> >> performance has been excellent with CIO. In the case where I switched
> from
> >> regular, buffered I/O to CIO and increased db_cache_size from 600M to
> 1500M,
> >> the performance of a fixed set of batch jobs improved from an average
> >> runtime of 166 minutes to 129 minutes - so a 22% reduction in runtime,
> but
> >> it's difficult to say how much of that improvement was from switching
> to CIO
> >> and how much was just due to the increase in db_cache_size alone.
> >>
> >> Here's a great paper specifically on AIX CIO for more info:
> >> http://www-03.ibm.com/servers/aix/whitepapers/db_perf_aix.pdf
> >>
>
> Chris Dunscombe
>
> www.christallize.com
>
> --
> http://www.freelists.org/webpage/oracle-l
>
>
>

-- 
Niall Litchfield
Oracle DBA
http://www.orawin.info

--
http://www.freelists.org/webpage/oracle-l
Received on Tue Jul 10 2007 - 08:42:12 CDT

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