RE: db file sequential/scattered read - physical or logical io or both?
- From: "Ric Van Dyke" <ric.van.dyke@xxxxxxxxxx>
- To: <oracledbawannabe@xxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 14 Jun 2010 09:02:40 -0500
If it's a scattered or a sequential read, then your server process is
requesting the block from the OS. What your process doesn't know is
where the block was gotten from.
You're correct that the fast times are "from cache" they are from a
cache at the system level, maybe the OS cache, the frame cache, or such.
Your process "thinks" it's a disk IO, but really it's a cache to cache
transfer of the block.
-----------------------
Ric Van Dyke
Hotsos Enterprises
-----------------------
Hotsos Symposium
March 6 - 10, 2011
You have to be there, yea I'm talking to you.
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Oracle Dba Wannabe
Sent: Sunday, June 13, 2010 6:45 PM
To: oracledbawannabe@xxxxxxxxx; oracle-l@xxxxxxxxxxxxx
Subject: Re: db file sequential/scattered read - physical or logical io
or both?
I suppose what I'm trying to say here is that if the same wait event is
issued regardless of a physical or logical io request, how can I
determine if the io subsystem is returning blocks at s reasonable
service time. Assuming an idle system other than the report, would times
under 5ms be cache hits and times over be physical. Further anything
over 10ms would be a bad service time ? appreciate any input on this
Thanks
On Mon, 14 Jun 2010 06:31 SGT Oracle Dba Wannabe wrote:
>Hi,
>So I have an idle system, where I run a sql report. The report takes an
hour to run. I look at the awr report (30 min interval), and see the io
wait events for sequential and scattered reads. The first has an avg
wait time of 7ms the second 10ms. These waits as I understand it are
physical io requests - correct? The p1 and p2 parameters point to file
and block numbers so I guess that makes sense. Anyway I rerun the same
report, look at the new awr and now see the same wait events, only with
much smaller wait times. Which means data was read from cache - if
that's the case why are the same wait events issued? it seems a bit
confusing that way.
>Thanks
>
>
>
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