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Date: Fri, 25 Jul 2014 07:45:23 -0600 (MDT)
From: Warren Block <wblock@wonkity.com>
To: "Patrick M. Hausen" <hausen@punkt.de>
Subject: Re: Gmirror + gpart corruption on 9.3-PRE
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On Fri, 25 Jul 2014, Patrick M. Hausen wrote:

> Am 25.07.2014 um 07:34 schrieb Warren Block <wblock@wonkity.com>:
>> True.  But partitioning can be specific to the drive.  It's not like GPT-partitioned drives can be copied with dd (well, not correctly).
>
> Errrr ... sorry, but ... could you please elaborate on that? Up until now I was sure that any drive could be
> copied with dd(1).
>
> Puzzled. :-)

GPT partitioning puts a primary partition table at the beginning of the 
drive, and a backup partition table at the end of the drive.  Use dd to 
copy a 500G drive to a 1TB drive, and that backup partition is now in 
the middle of the drive.  If the drives are even a single block 
different in size, that backup table will be in the wrong place, not 
found by the software and effectively lost.

This is dd's bug/feature: it does not understand the data being copied.

gpart(8) can back up and restore partition tables between 
differently-sized drives, and does it correctly.