Re: analyzing, visualizing, understanding and rating I/O latency

From: Marcin Przepiorowski <>
Date: Thu, 2 Aug 2012 11:43:27 +0100
Message-ID: <>

On Mon, Jul 30, 2012 at 7:47 PM, kyle Hailey <> wrote:
> Two questions I'm interested in answering or getting opinions on are:
> 1. what is considered great, good, ok and bad I/O and why? what do you
> use? latency values, throughput values, spread (ie like stdev), percentile
> latency

Hi Kyle,

I'm generally looking into latency - for most of system disk queue size and queuing time is a good indicator of performance problems.

> 2. is average latency good enough and if not why not? how would one use
> a latency histogram or 95-99.99 percentile latency to judge an I/O
> subsystem?

Generally average is good enough for data files but I use both average and histogram for redo log file systems. In both cases histograms can be useful to detect storage problems (like bad disk in array) - if you have millions of operations at 2 ms level
and a few operations at 1 s level due to disk error (but number of this operations is growing) it will take a while until average will be impacted
but it is easy to catch in histograms - I found v$event_histogram very useful in this area.


Marcin Przepiorowski
Received on Thu Aug 02 2012 - 05:43:27 CDT

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