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Date: Sun, 10 Mar 2013 13:34:48 -0600 (MDT)
From: Warren Block <wblock@wonkity.com>
To: Cody Ritts <cr@caltel.com>
Subject: Re: Aligning MBR for ZFS boot help
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On Sun, 10 Mar 2013, Cody Ritts wrote:

> So, aligning to 63MB was still tricky.  I found your thread, but I could not 
> find step by step how to calculate the offset.

Here is the procedure I had in mind:

# gpart create -s mbr da0
da0 created
root@lightning# gpart add -t freebsd -b 2016 da0
da0s1 added
# gpart show da0
=>      63  39070017  da0  MBR  (18G)
         63      1953       - free -  (976k)
       2016  39068064    1  freebsd  (18G)

# gpart create -s bsd da0s1
da0s1 created
# gpart add -t freebsd-zfs -a 1m da0s1
da0s1a added
root@lightning# gpart show da0s1
=>       0  39068064  da0s1  BSD  (18G)
          0        32         - free -  (16k)
         32  39067648      1  freebsd-zfs  (18G)
   39067680       384         - free -  (192k)

The first slice starts at the last CHS-aligned block before 1M, or 2016. 
Misaligned, but not a problem because nothing will be reading from that 
location.

The freebsd-zfs partition is created, letting gpart align it to 1M. 
gpart starts the partition at an offset of 32, making it the 1M-aligned 
block 2048 of the disk.

gpart should also be able to install the bootcode correctly, but I have 
not tried it for MBR and ZFS.