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Date:      Mon, 30 Sep 2002 19:32:33 -0500
From:      "Jack L. Stone" <jackstone@sage-one.net>
To:        Richard Tobin <richard@cogsci.ed.ac.uk>, "Pranav A. Desai" <pdesai1@cs.uh.edu>, <freebsd-questions@FreeBSD.ORG>
Subject:   Re: how to use 'dd' to create image of a hard drive?
Message-ID:  <3.0.5.32.20020930193233.028ece30@mail.sage-one.net>
In-Reply-To: <200210010027.BAA18320@sorley.cogsci.ed.ac.uk>
References:  <Jack L. Stone's message of Mon, 30 Sep 2002 19:10:06 -0500>

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At 01:27 AM 10.1.2002 +0100, Richard Tobin wrote:
>> bs=102400 or bs=128k will dd faster though.... you can play with that.
>
>That's not my experience.  8k dd runs at disk speed (30-40 MB/s) on
>all the machines I've used recently when copying between different
>disks on different ATA controllers.  Between disks on the same
>controller I only get about 13MB/s (why?) but there's no difference
>between 8k and 128k dd.
>
>-- Richard
>

Check your I/O. Try this:
dd if=/dev/ad0c of=/dev/null bs=128k &

and in the background do an 'iostat ad0 1'.  Here's an example with an
IDE drive:

# iostat ad0 1
        tty             ad0             cpu
   tin tout  KB/t tps  MB/s  us ni sy in id
     0    3  5.19   7  0.03  11  0  4  1 84
     0  126 127.36 183 22.74   0  0  6  2 92
     0   44 128.00 190 23.76   0  0  2  0 98
     0   44 128.00 191 23.89   0  0  5  0 95
     0   44 128.00 191 23.88   0  0  7  1 92

As you can see, it's really doing 128 kB transfers, for an average transfer
rate of almost 24 MB/s


Best regards,
Jack L. Stone,
Administrator

SageOne Net
http://www.sage-one.net
jackstone@sage-one.net

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