[Techtalk] dd confusion

[Conor Daly]

On Wed, Nov 05, 2003 at 06:24:25PM -0500 or so it is rumoured hereabouts, 
Travis Casey thought:
> On Wednesday 05 November 2003 14:28, Carla Schroder wrote:
> 
> > Two dd questions:
> >
> > I want to copy all the contents of hda to hdb. Does hdb have to be the
> > exact same disk geometry?
> 
> Well... maybe.  :-)
> 
> Using dd to copy all of one disk to another, as with
> 
>   dd if=/dev/hda of=/dev/hdb
> 
> copies *everything* on the disk... including, for some disk types and some 
> operating systems, data which tells what the disk geometry is.  That's 
> where the traditional warning to "only do this when the disk geometry is 
> the same" comes from.  

One thing that concerns me is the logical drive inside extended partition
bit.  AIUI, the logical drives are essentially a 'linked list'.  ie.
/dev/hda4 is an 'extended' partition and contains a partition 'table' that
describes /dev/hda5.  /dev/hda5 itself contains another 'partition table'
that describes /dev/hda6.  /dev/hda6 itself contains another 'partition table'
that describes /dev/hda7 except there is no /dev/hda7 so hda6 contains a
placeholder for hda7.  Now, if you start dd'ing these partitions around
the place, it's possible that things will get screwed up in ways that
_primary_ partitions wont.

Conor (cp -a)
-- 
Conor Daly <conor.daly at oceanfree.net>

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