Re: zero block writes

[Pat LaVarre <p.lavarre@ieee.org>]

> $
> $ sg_dd of=/dev/sg0 if=/dev/null bs=2k count=1
> 0+0 records in
> 0+0 records out
> $
> ...
> usb-storage:  2a 00 00 00 00 00 00 00 00 00

Kindly offline I'm told I neglected to emphasise, that example of an sg
utils sg_dd if=/dev/null command line differs only subtly from the
perhaps more familiar:

sg_dd of=/dev/sg0 if=/dev/zero bs=2k count=1

Copying from /dev/zero by design writes lots of zeroed bytes.

Copying from /dev/null via sg_dd doesn't neglect to write, in fact it
does write, it just tries to write nothing at a specific address.

Consider, for example:

sg_dd of=/dev/sg0 if=/dev/zero bs=2k seek=259 count=1

That changes the cdb trace so that the starting address passed thru as
cdb bytes[2:3:4:5] become x 00:00:01:03 rather than x 00:00:00:00:

x 2A 00 00 00 01 03 00 00 01 00

Pat LaVarre

P.S. Yes indeed I did stumble into this to-me-surprising fact via the
bonehead newbie error of typing "null" where I meant  "zero".