dd(1) performance when copiing a disk to another

[Tulio Guimarães da Silva]

Steven Hartland wrote:

> ----- Original Message ----- From: "Arne Wörner" <arne_woerner at yahoo.com>
>
>> That seems to be 2 or about 2 times faster than disc->disc
>> transfer... But still slower, than I would have expected...
>> SATA150 sounds like the drive can do 150MB/sec...
>
>
> LOL, you might want to read up on what SATA150 means.
> In short it the max throughput the interface can sustain. It is NOT
> what you can get of a single disk which is still fare from that,
> SATA disk transfer rates typically 30 -> 50MB/s sustained.
>
>    Steve

  Indeed. In other words, that represents the max transfer rates between 
the SATA controller and the disk´s controller (at best, you´ll get close 
to it when reading from the disk´s onboard cache), but the media will 
always be much slower.
  But just to clear out some questions...
  1) Maxtor´s full specifications for Diamond Max+ 9 Series refers to 
maximum *sustained* transfer rates of 37MB/s and 67MB/s for "ID" and 
"OD", respectively (though I couldn´d find exactly what it means, I 
deduced that represents the rates for center- and border-parts of the 
disk - please correct me if I´m wrong), then your tests show you´re 
getting the best out of it ;) ;
  2) Mr. Hartland mentioned the numbers to be good for a single drive, 
therefore it´s a bit better for a disk-to-disk, where the limit should 
be the slower disk´s performance. I couldn´t look for the specs of the 
Toshiba since I didn´t have the exact model, but I would expect it to be 
equal or faster than the Maxtor, since it does not appear to be a 
bottleneck.

  One last thought, though, for the specialists: iostat showed maximum 
of 128KB/transfer, even though dd should be using 1MB blocks... is that 
an expected behaviour? Shouldn´t iostat show 1024Kb/t, then?
  Thanks for your attention,

Tulio G. da Silva