Re: I/O and db_file_multiblock_read_count

As Mladen already noted, check if you ran dbms_stats.gather_system_stats. Ff you have system stats in place, Oracle won't use dbfmrc.

Stefan

On 12/8/06, Kevin Lidh <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:
>
> WAIT #1: nam='db file scattered read' ela= 21614 p1=6 p2=3979 p3=126 -->
> 126 blocks = 21614
> WAIT #1: nam='db file scattered read' ela= 10724 p1=6 p2=4107 p3=126
>
> WAIT #1: nam='db file scattered read' ela= 577 p1=6 p2=3979 p3=16
> WAIT #1: nam='db file scattered read' ela= 1524 p1=6 p2=3995 p3=16
> WAIT #1: nam='db file scattered read' ela= 916 p1=6 p2=4011 p3=16
> WAIT #1: nam='db file scattered read' ela= 1022 p1=6 p2=4027 p3=16
> WAIT #1: nam='db file scattered read' ela= 1095 p1=6 p2=4043 p3=16
> WAIT #1: nam='db file scattered read' ela= 1026 p1=6 p2=4059 p3=16
> WAIT #1: nam='db file scattered read' ela= 829 p1=6 p2=4075 p3=16
> WAIT #1: nam='db file scattered read' ela= 826 p1=6 p2=4091 p3=14 -->
> 126 blocks = 7815
> WAIT #1: nam='db file scattered read' ela= 437 p1=6 p2=4107 p3=16
>
> 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?
>

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