Re: Speed of dd depends on skip/seek values?

On 20090110T2323, "Butler, Jeff" wrote:

> > Examples, with "if=/dev/sda of=/dev/sdb" omitted for brevity:
> > - 750 GB: dd bs=8192 count=91571823 takes 07h12m
> > - 535 GB: dd bs=1440256 count=371567 skip=149282 seek=43831 takes
> > 07h08m
>
> First, as you move to the inner tracks of a disk, performance will drop.
> That's what seek/skip will cause to happen.  Disks will vary, but it
> would not surprise me to see the bandwidth drop in half when you move to
> the inner tracks.

Thanks for your answer.

I think, however, that this is not what's happening. The first command
encompasses the second one, both on the source disc and on the target
disc.

That is, the first command /reads/ all sectors that are also read in
the second command, and then some more. Same for the target disc: the
first command /writes/ all sectors that are also written in the second
command, and then some more.

I therefore expect the first command to take significantly more time
than the second one, as it has almost half as many sectors to
transfer.

Rotaluclac