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Re: Interpretating ela= values for "db file sequential read" in trace files

From: IANAL_VISTA <IANAL_Vista_at_hotmail.com>
Date: Sat, 20 Nov 2004 01:53:48 GMT
Message-ID: <Xns95A6B60E9EB48SunnySD@68.6.19.6>


BigBoote66_at_hotmail.com (Steve B) wrote in news:67bcf80a.0411191602.28d574d_at_posting.google.com:

> Hi all,
>
> I'm in the process of trying to chase down a disk i/o problem with our
> Oracle 9 instance on Solaris 8, and I've been looking at the 10046
> trace files for the relevant sessions. One thing that we've
> discovered is that our critical processes are running much faster when
> we are setting DISK_ASYNCH_IO is set to FALSE, at least according to
> the trace files.
>
> Specifically, we have times when some of our select statements have
> "db file sequential read" times exceeding 4 seconds (with asynch i/o
> turned on), but the highest we ever get when it is turned off is 40
> milliseconds.
>
> What I'm wonder is: could this be an illusion? I'm reading Millsap's
> "Optimizing Oracle Performance", and one thing I'm unsure about: how
> can the wait times reported for "db file sequential read" be
> cumulative if you are running with asynch_io turned on? In other
> words, if you had a statement that made 100 dfsr requests and did them
> all at once, it would then proceed to wait as each one came back. If
> the average time to service a request was 20 ms, then you'd probably
> see one that waited 20, one that waited 40, one that waited 60 & so
> on, up to one that waited 2000 ms.
>
> But if you were running with asynch turned off, you'd just see 100
> requests each taking 20. This would make the asynch reporting seem
> slower, when in reality they took the same amount of time (or the
> asynch might be even faster).
>
> Is there something about the reporting of disk i/o that I'm not
> understanding when it comes to these files?
>
> -Steve

Cary is a frequent contributor to the ORACLE-L email list. Is supect that if you post your question there, you'll it answered. Received on Fri Nov 19 2004 - 19:53:48 CST

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