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  1. #1
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    Sep 2006
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    Unanswered: DB parameter....

    Hi.......all

    i want to know about the db parameter....default prefetch size.......

    In db2 get db cfg command,it is showing........

    Default prefetch size (pages) (DFT_PREFETCH_SZ) = AUTOMATIC

    (DFT_EXTENT_SZ) = 32.

    The prefetch size may be half the extent size or multiples of the extent size...right?

    then what does AUTOMATIC refers to.........

    what size DB2 will take......

    THANKS.

  2. #2
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    Meena
    If the parameter is set to AUTOMATIC, DB2 calculates the value based on the following equation
    prefetch size = (# containers) * (# physical spindles) * extent size

    The prefetch size is calculated at database startup time and when a tablespace is created or altered

    HTH

    Sathyaram
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  3. #3
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    BTW, where did you read that the prefetch size may be half the extent size ?

    Sathyaram
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  4. #4
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    Sep 2006
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    Hi.....

    Actually i read this from DB2 information center.From the following site....

    http://publib.boulder.ibm.com/infoce...dex.jsp.Thanks for ur information.

    Thanks
    Meena

  5. #5
    Join Date
    Mar 2003
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    343

    Question

    Quote Originally Posted by sathyaram_s
    Meena
    If the parameter is set to AUTOMATIC, DB2 calculates the value based on the following equation
    prefetch size = (# containers) * (# physical spindles) * extent size

    The prefetch size is calculated at database startup time and when a tablespace is created or altered

    HTH

    Sathyaram
    How does db2 determine the number of physical Spindles?

  6. #6
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    Search for DB2_PARALLEL_IO in the documentation

    Sathyaram
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  7. #7
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    The number of physical spindles defaults to 1 and can be specified through the DB2 registry variable DB2_PARALLEL_IO.

    Thanks
    Meena

  8. #8
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    The number is 1 if you have not enabled parallel io

    If you have, IIRC, the number is 6 ..

    For most 'modern' systems, you will have to set parallel io for exploiting the IO capability

    Sathyaram
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  9. #9
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    I understood that setting DB2_PARALLEL_IO=6 enables parallel io for tbsp whose tablespace-id is 6. If it is set to * enables parallel io for all tablespaces.

    When I read your response I thought that maybe IBM changed what it meant in subsequent versions so I checked IBM's website and the definition of this registry variable does not appear to have changed.

    So the question remains - how is db2 informed of the number of physical spindles in the RAID group.

  10. #10
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  11. #11
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    Mar 2003
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    It does, thanks.

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