Re: I/O and db_file_multiblock_read_count

The thing is, when you collect system statistics (aka make use of cpu costing) - Oracle gathers stats about your systems' average multiblock read count. It then uses that information instead of whatever you might have set in your init.ora.

Therefore, his test may be wrong - IF he's got sysstats.

Stefan

On 12/9/06, Hemant K Chitale <hkchital_at_singnet.com.sg> wrote:
>
>
> I don't undersetand Mladen's and Stefan's recommendations about
> running dbms_stats.gather_system_stats.
>
> Kevin says that he deliberately ran a test to check the performance
> of multiblock reads.
> He is not asking "how can I avoid multiblock reads ?".
> He is asking "why is the total time for 128-block reads not
> significantly lesser than the total time for 16-block reads ?".
>
> Hemant
>
>
>
> >On 12/8/06, Kevin Lidh
> ><<mailto:kevin.lidh_at_gmail.com>kevin.lidh_at_gmail.com> wrote:
> >I was reading an article about the appropriate setting for
> >db_file_multiblock_read_count. I'm on a HP-UX 11.11 64-bit system
> >with Oracle 9.2.0.7.0. The original value was 16 and I bounced the
> >database and ran a million record full-scan test (10046 trace) and
> >then set the value to 128 (max value) and re-ran the test. Although
> >I did have less calls to 'db file scattered read' (140 compared to
> >1008), the time to get the same number of blocks was longer. A
> >random example from the trace files looks like this:
> ><snip>
> >And I ran the test again with 16 after my 128 test and the results
> >were similar to the first test. The cumulative times for the 'db
> >file scattered read' was 1.3181s for the 16 and 2.5149s when it was
> >128. We use a Hitachi SAN and I know it has caching but I would
> >still think that the fewer, bigger requests would be faster. Is
> >there a measurement or setting on the operating system or even SAN
> >that I can look at to help determine the optimal setting?
> >
>
>
> Hemant K Chitale
> http://web.singnet.com.sg/~hkchital
>
>
>

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