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Date:      Fri, 11 Dec 2009 14:57:49 -0500
From:      Michael Powell <>
Subject:   Re: use DD mode or not? and how to set up?
Message-ID:  <hfu882$96o$>
References:  <>

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Tom Worster wrote:

> options for setting up gmirror include DD or standard mode. i don't know
> about others.
> i'm about to install 8.0-RELEASE on a system with two 750g sata disks that
> i want to run as a mirrored pair.
> what are the pros/cons of the different options?
> and what about the installation process? set up the mirror before
> installing from the CD or do it like the handbook says?

I believe DD mode is going away. Probably has something to do with adding 
features to loader for GPT support. A GPT places what's called a 
"protective" MBR in the place where the old DOS style MBR would normally 
occupy. I would avoid DD.

Last question is easy if dealing with a true hardware RAID card, as you will 
create the array in the card's BIOS and FreeBSD will just see that as ar0 
and install to that.

As far as a perfect answer I'm not certain. There is one little tidbit that 
may assist though. The metadata gets stored in the last sector at the end of 
an empty disk. As long as there is free space at the end of the drive(s) it 
won't matter which way you do it.

The Handbook is essentially reconfiguring an already virgin install existing 
on the first drive. When rebooted into this changed configuration, the 
second drive is then added to the mirror and replication will take place. 
The less amount of data present will speed this replication and get you to 
mirrored status quicker. I've done this a couple of times and it was pretty 


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