High mbrc in aux_stats$ 2006-03-12 - By oracle-l-bounce@(protected)
Back
Ciao Luca
>On a related note, Oracle 10gR2 has changed the default value on
>db_file_multiblock_read_count to match the system I/O max size (on my
>test Linux DBs with 8K block I ended up with
>db_file_multiblock_read_count=128).
Attention, it is not the max I/O size, it is, according to the reference guide,
the "maximum I/O size that can be efficiently performed". It seams not that
different... in reality it is!
>Oracle's documentation seems to me somewhat cryptic on this. First they
>say:
>
>"Even though the default value may be a large value, the optimizer will
>not favor large plans if you do not set this parameter. It would do so
>only if you explicitly set this parameter to a large value."
This is because in 10gR2 either MBRC (system statistics) or a default value (8)
is used.
>...any thoughts/experiences on using the default value for
>db_file_multiblock_read_count on 10gR2?
DON'T USE IT. Presently the "auto-tuned" value is not dependent on performance
figures but only on a couple of other parameters.
HTH
Chris
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