Re: on "dd" using "seek" vs. "read" & >32-bit sizes

[Pádraig Brady]

Linda Walsh wrote:

> Sometimes I would like "dd" to copy the contents of
> "Random-Block-Object-A"
> to "Random-Block-Object-B", starting from the middle of of the file.
>
> I can specify the "seek=BLOCKS" param, but it appears that "dd" doesn't
> translate this to a "seek" command but instead uses "read" to to
> sequentially access the block device.  Is this the case or am I
> hallucinating?

I think you are hallucinating. Put an strace before your dd command to
confirm.
seek is a pretty fundamental feature of a block device :)

>
> If I am not hallucinating, I can see this being done by design
> from a historical perspective, working with tapes or such, but I'm
> not sure it makes as much sense today.  Perhaps an attempt at an lseek
> on an "OS", and if that fails, then failover to the older "read"
> behavior?
>
> If it is deemed necessary, maybe some flag to force old behavior?
>
> It gets painful when copying multi-gigabyte images where one may have
> already copied previous parts of an image.
>
> Second question I had -- at times, it seems like various utils
> are limited to 2G file offsets on OS's (i686-linux, i686-cygwin)
> that support files >2G.  Is this "old" behavior that will (should?)
> be disappearing?  I'd noticed it before in specifying counts
> and such in "dd" (among others).

What platform, filesystem and version of dd are you using?
I've confirmed on my fedora core 3 & ubuntu 5.10 systems
that I can seek to 2TB with the command `truncate 2TB ext3.test`:
http://www.pixelbeat.org/scripts/truncate

Pádraig.