Re: db file sequential/scattered read - physical or logical io or both?
- From: Niall Litchfield <niall.litchfield@xxxxxxxxx>
- To: oracledbawannabe@xxxxxxxxx
- Date: Mon, 14 Jun 2010 06:55:46 +0100
On Sun, Jun 13, 2010 at 11:45 PM, Oracle Dba Wannabe <
oracledbawannabe@xxxxxxxxx> wrote:
> 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
>
Hi, the sequential and scattered read requests *always* represent physical
i/o *requests*, that is they correspond directly to requests *outside* of
the Oracle instance for a filesystem or volume manager disk access. Since
there is at least one cache in between Oracle and the disk surface - and
often 2 - these requests can be satisfied from a cache outside of Oracle and
so complete quickly. Oracle reserves the term 'logical i/o' for all Oracle
i/o operations, that is all *Oracle* calls to obtain one or more blocks.
This includes, hopefully many, occasions when the request is for a buffer in
the buffer cache.
--
Niall Litchfield
Oracle DBA
http://www.orawin.info
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