How much RAM does your machine have?

Write to the disk an amount that is larger than the amount of RAM and
see if you get the same result.

Also you can use hdparm to flush the caches before running dd the 2nd time.

2010/7/1 Shaochun Wang <scwang@ios.ac.cn>
quoted text
>
> On Wed, Jun 30, 2010 at 07:54:55AM +0200, Mikael Abrahamsson wrote:
> > On Wed, 30 Jun 2010, Roman Mamedov wrote:
> >
> > >  dd if=/dev/zero of=zerofile bs=1M count=2048 conv=fdatasync,notrunc
> >
> > To be a bit more sure about accurate measurement, make sure the amount of
> > data you're transferring is twice the amount of RAM in the machine, the
> > above is only 2 gigs of data.
> As suggested, I did the test again and got the following result:
>
>        -bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000 conv=fdatasync,notrunc
>        Password:
>        5000+0 records in
>        5000+0 records out
>        5242880000 bytes (5.2 GB) copied, 63.497 s, 82.6 MB/s
>
>        -bash-4.1$ sudo dd if=/dev/zero of=test.dd bs=1M count=5000 conv=fdatasync,notrunc
>        5000+0 records in
>        5000+0 records out
>        5242880000 bytes (5.2 GB) copied, 18.1033 s, 290 MB/s
>
> I don't know why the second dd becomes 290MB/s and the first 82.6MB/s.
>
> --
> Shaochun Wang(王绍春) <scwang@ios.ac.cn>
> State Key Laboratory of Computer Science,
> Institute of Software,
> Chinese Academy of Sciences
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