Re: RAID-1 performance under 2.4 and 2.6

[Bill Davidsen]

Bart Van Assche wrote:
> On Thu, Mar 27, 2008 at 10:53 PM, Bill Davidsen <davidsen@xxxxxxx> wrote:
>   
> > Bart Van Assche wrote:
> >     
> > > On Tue, Mar 25, 2008 at 11:00 PM, Chris Snook <csnook@xxxxxxxxxx> 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@xxxxxxx>
 "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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