linux-kernel - Re: [Patch 0/5] I/O statistics through request queues

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Date:	Wed, 25 Oct 2006 00:27:52 +0200
From:	Martin Peschke <mp3@...ibm.com>
To:	Phillip Susi <psusi@....rr.com>
CC:	Jens Axboe <jens.axboe@...cle.com>, Andrew Morton <akpm@...l.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [Patch 0/5] I/O statistics through request queues

Phillip Susi wrote:
> This discussion seems to involve two different solutions to two 
> different problems.  If it is a simple counter you want to be able to 
> poll, then sysfs/debugfs is an appropriate place to make the count 
> available.  If it is a detailed log of IO requests that you are after, 
> then blktrace is appropriate.

It's about counters ... well, sometimes a buch of counters called
histogram.

> I did not read the patch to see, so I must ask: does it merely keep 
> statistics or does it log events?  If it is just statistics you are 
> after, then clearly blktrace is not the appropriate tool to use.

If matters were as simple as that, sigh.

Statistics feed on data reported through events.
"Oh, this request has completed - time to update I/O counters."

The tricky question is: is event processing, that is, statistics data
aggregation, better done later (in user space), or immediately
(in the kernel). Both approaches exist: blktrace/btt vs.
gendisk statistics used by iostat, for example.

My feeling was that the in-kernel counters approach of my patch
was fine with regard to the purpose of these statistics. But blktrace
exists, undeniably, and deserves a closer look.

Martin

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