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SubjectRe: RAID-1 performance under 2.4 and 2.6
Bart Van Assche wrote:
> On Thu, Mar 27, 2008 at 10:53 PM, Bill Davidsen <> wrote:
>> Bart Van Assche wrote:
>>> On Tue, Mar 25, 2008 at 11:00 PM, Chris Snook <> 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 <>
"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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