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Date: Wed, 14 Jan 2004 14:03:46 -0500 (EST)
From: Robert Watson <rwatson@FreeBSD.org>
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To: Kevin Oberman <oberman@es.net>
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cc: Dan Nelson <dnelson@allantgroup.com>
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cc: John Baldwin <jhb@FreeBSD.org>
Subject: Re: Serious dd breakage in current 
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On Wed, 14 Jan 2004, Kevin Oberman wrote:

> > dump / restore is a better method though anyways.  It doesn't bother with 
> > copying unused blocks for one thing.  It will also allow things like dirhash 
> > to more efficiently lay out your files on the new file system.
> 
> Thanks to both John and Dan. I clearly did not correctly understand how
> dd operated. Guess it's time to read the sources a bit. 
> 
> I understand why dump|restore is a better choice than dd in may ways,
> but, if a partition is large and full, dd is MUCH faster. That's why I
> use it to backup my system disk. I can copy 40 GB in about 40 minutes on
> my laptop and dumping takes just a bit longer. 
> 
> Non the less, now that I see the problem with dd on a partition, I have
> used dump and my system is now properly re-partitioned. It's nice to see
> both /var and /usr under 98%! 
> 
> Thanks again for taking the time to explain this.

FWIW, you can actually dd the individual partitions fine, since the file
system doesn't use absolute offsets from the slice, just from within the
partition.  You just want to not dd the partition table to a drive that
isn't laid out the same (at least, not without being very careful). 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org      Senior Research Scientist, McAfee Research