Re: db file sequential/scattered read - physical or logical io or both?
Date: Mon, 14 Jun 2010 06:55:46 +0100
Message-ID: <AANLkTim4MhmXwE7Dja2s0TL-_NrVASgLvx-vx6ssjBEF_at_mail.gmail.com>
On Sun, Jun 13, 2010 at 11:45 PM, Oracle Dba Wannabe < oracledbawannabe_at_yahoo.com> 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 -- http://www.freelists.org/webpage/oracle-lReceived on Mon Jun 14 2010 - 00:55:46 CDT