Re: Question regarding db_file_scattered_read

  • From: Dennis Williams <oracledba.williams@xxxxxxxxx>
  • To: GJohnson@xxxxxxx
  • Date: Mon, 18 Jul 2005 08:40:44 -0500


    George, congratulations on thinking ahead, anticipating problems
before your users encounter them. Of course, you'll get no credit from
management, but such is the DBA's lot.
    In Oracle terms, db_file_scattered_read usually equates to a full
table scan. Start looking for queries that are executing a full table
scan. Following Pareto's rule (80/20 principle), you probably have a
few queries that are doing a FTS on your largest tables. Tame them and
your system performance should look better.

Dennis Williams

On 7/18/05, Johnson, George <GJohnson@xxxxxxx> wrote:
>         I have a DB on a Solaris box, the system that is almost at 100%
> utilisation, I'm not overly worried as no one is complaining, yet. I am
> simply trying to identify where the resources are being consumed, partly
> curiosity and partly in the hope of preventing a potential future problem. 
>         I have been over the wait stats and found that the system spends
> it's time, almost exclusively, making users wait on db_file_scattered_read,
> about 30% of their session time, in fact. However the OS reports hardly any
> I/O wait time, the system resources are being used up by user CPU. I believe
> the entire SGA is little too small, I am wondering if the scattered_read
> waits are a result of the SGA and/or buffer cache being too small or would a
> buffer cache that is too small, more likely result in buffer_busy_waits or
> have I got my thinking completely backside-about-face!? All the notes I can
> find relate db_file_scattered_read to an I/O performance problem, which from
> what I can tell from the OS, I don't seem to have.
>         Any pointers in the direction I should start off in next, would be
> greatly appreciated. 
>         Rgds 
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