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

Qihua,

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:

http://www.quest-pipelines.com/newsletter-v3/0402_C.htm Tuning Oracle Without Cache-Hit Ratios

Of particular interest with regard to your question is the section, IDENTIFYING THE OS BOTTLENECKS - PRONG II. You may also want to read the following post that shows a correlation between the OS-level IO waits and those reported in Statspack.

http://www.freelists.org/archives/oracle-l/11-2005/msg00411.html High wio on New Hitachi SAN Storage of DB Server

Hope this is of some help.

Cheers,

David

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

--
http://www.freelists.org/webpage/oracle-l