[ale] Understanding dd and/or /dev/zero

[danscox at mindspring.com (Danny Cox)]

Greg,

On Mon, 2005-01-10 at 16:54 -0500, Greg Freemyer wrote:
> I've noticed that I get a log of disk read activitity on /dev/hdc when I do 
> 
>     dd if=/dev/zero of=/dev/hdc
> 
> I'm using "iostat -d 5" to see the read activity, and it is really
> slowing down the above.
> 
> I have no idea why there is disk read activity, and the machine is
> doing nothing else.  Also if I kill the above dd, the read activity
> stops.

	I don't know why you're seeing reads either, unless iostat is wrong, or
the kernel is.

	As another data point, try bumping up the block size of the dd like
this:

	dd bs=1024M if=/dev/zero of=/dev/hdc

and stand well back ;-).

-- 
kernel, n.: A part of an operating system that preserves the
medieval traditions of sorcery and black art.

Danny