Mailinglist Archive: opensuse (1986 mails)

[Randall R Schulz <rschulz@xxxxxxxxx>]

On Thursday 20 November 2008 00:30, Per Jessen wrote:
> Randall R Schulz wrote:
> > On Wednesday 19 November 2008 10:12, Per Jessen wrote:
> > > > dd if=/tmp/file1 of=/tmp/file2 bs=1M
> > >
> > > 27m28s  13Mb/sec
> > >
> > > > dd if=/tmp/file1 of=/tmp/file2 bs=1000M
> > >
> > > 17m34s  20Mb/sec
> >
> > That's not really a correct computation. Every byte (or sector or
> > whatever) must be read once and written once, and you have to
> > account for both the read and the write traffic.
>
> I didn't compute it, I was just quoting from the dd output.

Yes, but dd is only telling you its throughput. Getting the correct I/O 
device throughput requires that you know things that dd does not about 
where the data is coming from and going to.

In this case, you know the input and the output both originate and 
terminate in the same disk drive, so that drive (and the buss that 
connects it to its controller) are serving both the read and the write 
traffic.

So unless you account for this, you're underestimating the device (and 
buss and controller) throughput by a factor of two.


> /Per


Randall Schulz
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