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Date: Sun, 10 Mar 2013 18:39:30 -0600 (MDT)
From: Warren Block <wblock@wonkity.com>
To: Dennis Glatting <freebsd@penx.com>
Subject: Re: Aligning MBR for ZFS boot help
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On Sun, 10 Mar 2013, Dennis Glatting wrote:

> Sorry for the stupid question but is this issue (and issues) and
> procedures written up somewhere?

The wiki shows using GPT:

https://wiki.freebsd.org/RootOnZFS/GPTZFSBoot/9.0-RELEASE

The issue with MBR is that FreeBSD strictly follows the standard of 
alignment to CHS values.  However, for the last couple of decades, 
drives have used variable geometry to fit more data on the outside 
tracks, so CHS values don't apply any more.  Combine this with the new 
need to align MBR slices to particular values of 4K or 1M, and FreeBSD 
has a problem.

Solution: use GPT when possible.  But some systems won't boot from GPT.

If MBR partitioning is required, and alignment is needed, use some other 
operating system to create the MBR, and don't try to edit the slices on 
FreeBSD.

I don't know if anyone has documented all this in one place.  If FreeBSD 
had a way to turn off the strict enforcement of CHS values for MBR, it 
would make that unnecessary.