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Date: Fri, 28 Nov 2008 18:35:14 +0100 (CET)
From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To: Andrew Falanga <af300wsm@gmail.com>
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References: <200811281008.19578.af300wsm@gmail.com>
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Cc: FreeBSD Questions <freebsd-questions@freebsd.org>
Subject: Re: gmirror and the UFS file systems
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> I'm getting ready to move forward on enabling gmirror on my churches website
> server (FreeBSD 7.0-RELEASE p4).  I used defaults during the install (most
> importantly for this, the file system defaults).  I've read in the manual
> pages that the data for the mirror is contained in the last sector of the
> drive/partitions.  So, I want to mirror the entire drive (ad4) to the second
> drive (ad5).  This server doesn't yet have much data at all.  I'm wondering
> if I turn on this mirror, will anything important be overwritten in the last
> sector?
is your partition size multiply of fragment size without remainder?

if not (quite a big chance) at least one sector at the end is unused and 
never be.
so go on, but then fix disklabel, as c partition is 1 sector smaller.

of course - boot from livecd to do this.