[1379] in RedHat Linux List
Re: dd to copy
daemon@ATHENA.MIT.EDU (Paul Anderson)
Mon Oct 28 07:17:26 1996
Date: Mon, 28 Oct 1996 07:31:43 -0500
From: Paul Anderson <andersop@agapesystems.com>
To: redhat-list@redhat.com
CC: Mike Sangrey <mike@sojurn.lns.pa.us>
Resent-From: redhat-list@redhat.com
Reply-To: redhat-list@redhat.com
redhat-digest-request@redhat.com wrote:
> Subject: Re: dd to copy
> Date: Sun, 27 Oct 1996 19:35:44 -0500
> From: Mike Sangrey <mike@sojurn.lns.pa.us>
> To: redhat-list@redhat.com
>
> William T Wilson wrote:
> > > This statement is inaccurate. The drives do not need to be the same
> > > size and have the exact same geometry. dd is a copy at the bit level
> >
> > Okay, perhaps they do not need the same geometry. They do however have to
> > be the same size. Suppose the destination drive is SMALLER than the
> > source drive. Then when dd tries to write out the data, it will write
> > past the end of the device. If the destination drive is LARGER, then the
> > filesystem which is created will think it is smaller than it is, which
> > will drive everything nuts.
>
> Yep! For amusement, try
> dd if=/dev/fd0 of=/dev/hda1
>
> where /dev/fd0 has a filesystem on it. You now have a 1.4M filesystem (less
> overhead) on /dev/hda1. This extreme example makes the problem obvious. Hard
> drives that are nearly the same could easily produce disastrous results; the
> problem not being obvious till it's too late.
>
> Using dd in this way is just too dangerous -- there is just too much
> non-intuitive affects that could cause something to go wrong when using this
> as a general purpose solution. Best to just stick with tar or cpio. dd
> should be thought of as making a physical image copy. Not for copying files
> or even a filesystem.
>
> It might be useful after a mkisofs. Has anyone tried using dd to copy the
> CDROM image to a CDROM writer? This is a case of physical image copy and this
> is where dd shines.
>
> --
> Mike Sangrey <mike@sojurn.lns.pa.us> (Home)
> <Mike.Sangrey@specmarkmet.com>(Work)
> "I've trademarked `William Della Croce, Jr.(tm)'.
> Anyone using this name owes me $1,000,000."
>
OK, I guess I did forget to mention that with any Unix based command,
including dd, one has to be less than brain dead in order to use it.
Your example would denote a brain dead condition to even try it. I
really don't have a problem with cpio or tar, although personally I tend
to use cpio. The example I first responded to was to copy the partition
of a filesystem on one disk to the partition of a newly formatted disk.
In this situation I believe my statements to be correct. I did not
formulate the thesis that all copies should be made by using dd, and
certainly the situtaion you propose was never a part of this thread.
Paul Anderson
--
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