Re: Db file scattered reads vs. direct path reads

  • From: "Finn Jorgensen" <finn.oracledba@xxxxxxxxx>
  • To: Josh.Collier@xxxxxxxxxxxx
  • Date: Fri, 22 Feb 2008 10:01:13 -0500

What type of filesystem are you on? What differences are there between the
old system and the new apart from the storage array?

Finn


On 2/20/08, Josh Collier <Josh.Collier@xxxxxxxxxxxx> wrote:
>
>  Hi,
>
> I've got a conundrum. We've got a new disk system. When I full scan a
> table with the nocache hint the trace files record lots of reads like this
>
> WAIT #1: nam='db file scattered read' ela= 38471 file#=777 block#=238789
> blocks=64 obj#=2805234 tim=359927798919
>
> And the query takes 11 minutes for a 19gb table, much slower than our old
> system
>
> When I enforce direct path reads with the following parameter
> :alter session set "_serial_direct_read" = true
>
> The trace files record lots of reads like this
>
> WAIT #1: nam='direct path read' ela= 5 file number=777 first dba=238789
> block cnt=64 obj#=2805234 tim=343398354853
>
> And the query returns in 3 minutes
>
> What would cause the scattered reads to be so much slower than the direct
> path reads?
>
> I've enabled direct io on the db and verified it via truss.
>
> Version is 10.2.0.1
> Solaris 10
>
>
>
>
>
>

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