linux-kernel - Re: RAID-1 performance under 2.4 and 2.6

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Date:	Fri, 28 Mar 2008 08:04:38 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Bart Van Assche <bart.vanassche@...il.com>
CC:	Chris Snook <csnook@...hat.com>,
	Emmanuel Florac <eflorac@...ellique.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RAID-1 performance under 2.4 and 2.6

Bart Van Assche wrote:
> On Thu, Mar 27, 2008 at 10:53 PM, Bill Davidsen <davidsen@....com> wrote:
>   
>> Bart Van Assche wrote:
>>     
>>> On Tue, Mar 25, 2008 at 11:00 PM, Chris Snook <csnook@...hat.com> wrote:
>>>       
>>>>  It means you shouldn't use dd as a benchmark.
>>>>         
>>> If you want to benchmark write speed, you should add
>>> oflag=direct,dsync to the dd command line. For benchmarking read speed
>>> you should specify iflag=direct. Or, even better, you can use xdd with
>>> the flags -dio -processlock.
>>>       
>>  No, you want your benchmark to measure performance doing what the
>>  application does. Do unless you have an application which has been
>>  heavily Linux-ized you don't want to measure something unrelated to the
>>  application requirements.
>>     
>
> A basic fact I learned in science classes: if you measure something,
> know very well what you measure and make sure your measurement is
> repeatable. But it was some time ago I learned this. Maybe the whole
> world changed since I learned that ?
>   

Sounds like we're saying the same thing. For naive applications dd is 
probably a closer model without direct or fconv, while if you want to 
see what you could gain using additional features those are useful 
options. I believe Chris was talking about the max speed possible, which 
is a good thing to know but not similar to simple programming or shell 
scripts using sed, grep, etc.

-- 
Bill Davidsen <davidsen@....com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark 


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