From owner-freebsd-hackers@FreeBSD.ORG  Thu Jan 12 13:04:10 2006
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Date: Thu, 12 Jan 2006 14:01:38 +0100
From: Ivan Voras <ivoras@fer.hr>
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To: Christoph Kukulies <kuku@www.kukulies.org>
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Cc: hackers@freebsd.org
Subject: Re: increasing dd disk to disk transfer rate
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Christoph Kukulies wrote:

> Anyway, I decided to buy a second identical hard disk and tried to
> block by block copy the old disk to the new one using
> 
> dd if=/dev/ad2 of=/dev/ad3 conv=noerror
> 
> The process is running now since yesterday evening and it is at 53 MB
> at a transfer rate of about 1.1 MB/s.

The default block size for dd is 512 bytes, meaning dd will read 512 
bytes from one disk and write them to the other before reading again. 
This is SLOW. You need to specify a larger block size to use it 
effectively, like adding "bs=1m" argument to dd (which will make it use 
1 MB blocks). Also, you should probably add "sync" to your "conv" 
argument, see the manual page of dd. I don't know if using "sync" will 
produce a full 1 MB of zeros when a bad sector is encountered - I hope 
someone else will clarify this :)

Btw. I don't think this is the right group for your question - in the 
future use questions@freebsd.org or stable@freebsd.org...