Re: what's the difference between "CPU I/O Wait" and "db scatter/sqenecail wait"

  • From: "David Taft" <oradbt054@xxxxxxxxx>
  • To: staywithpin@xxxxxxxxx
  • Date: Mon, 1 Oct 2007 11:04:29 -0400


Others are more knowledgeable than myself on this, but I will give this
a shot.  As I understand it, whenever Oracle is waiting on IO, the
instrumentation is such that this time is reported to OS kernel. Hence any
IO waits in Oracle are also reported at the OS level, but not all IO waits
at the OS level are necessarily correlated to Oracle IO waits.  When an
Oracle process is waiting on IO to complete, it voluntarily takes itself off
the CPU since it can't continue processing until the required IO completes.
The Unix kernel requires this info from Oracle processes for CPU time slice
scheduling.  If I got any of this wrong, please someone correct me.  Anyway,
you may find the following article worth reading:
Tuning Oracle Without Cache-Hit Ratios

Of particular interest with regard to your question is the section,

You may also want to read the following post that shows a correlation
between the OS-level IO waits and those reported in Statspack.
High wio on New Hitachi SAN Storage of DB Server

Hope this is of some help.



On 9/29/07, qihua wu <staywithpin@xxxxxxxxx> wrote:
> the same or at least related to the "CPU I/O wait' at the unix level?

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