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Date:      Thu, 19 Nov 2009 04:01:34 +0100
From:      Rolf Nielsen <>
To:        Glen Barber <>
Subject:   Re: Loader does not load after reinstall with previous mirrored disk attached
Message-ID:  <>
In-Reply-To: <>
References:  <>

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Glen Barber wrote:
> Hello,
> First off, this is not a real "I need help" post, though there are a
> few questions that I'd like to get insight on, if possible - it is
> more for informative purposes in the archives, should someone run into
> this very random problem in the future. Additionally, I believe this
> is far too unlikely this will happen to many others to consider it to
> be a "problem", thus submitting a PR.
> The story, cut down from its original 3 hour length:
> For some time, I was dual-booting OpenSolaris and FreeBSD, on 500GB
> SATA and 250GB IDE disks, respectively.  When I lost the need to run
> OpenSolaris (Flash), I purchased a second 500GB SATA disk.  I added
> the original SATA disk to gmirror, and synced the 250GB FreeBSD
> install to the 500GB SATA drive.  Once that finished, I removed the
> 250GB disk, and installed the second 500GB disk, adding it to the
> mirror.
> All was fine, except I was not using all of the drive space available.
>  This past weekend, I decided to create a slice to cover the remaining
> 250GB, initially to try ZFS, which failed for the following reason.
> Here is where question 1 comes into play.
> 1.)  What is the correct way to completely break a gmirror setup,
> replacing /dev/mirror/gm0 with the original /dev/adN configuration?
> Is the assumption that once you initialize gmirror, you will never
> need/want to go back to a single disk setup?  Is there a clean way to
> fix boot0 so it does not look for a mirror, short of
> 'vfs.root.mountfrom' in loader.conf?
> After rendering my system unbootable since I use a USB keyboard, and
> was not able to do anything at the 'mountroot>' prompt, I decided it
> may be a good time for me to finally upgrade to 64-bit, where I
> purchased an additional 4GB RAM and burned a bootonly amd64 disc.  It
> wasn't a big deal for me since I knew my data was mirrored and
> available.  However, I did consider purchasing a PS/2 keyboard to fix
> the system... But, let's be honest, who can resist an upgrade? :-)
> Here is the real reason for my post (besides archival purposes).
> Because I never disconnected the secondary disk (from the original
> mirror), it caused the loader to hang upon boot - no errors, just a
> blinking cursor on the top-left side.  One answer I received about
> this, which makes perfect sense considering what I saw when I was
> trying to figure out what was happening (until I disconnected the
> second disk to avoid writing over it accidentally), is that the new
> loader and the previous loader were in conflict at boot.  Once the
> secondary disk was disconnected, the system booted fine.  Which brings
> me to my second question:
> 2.)  loader problem?  In hindsight, I can expect the loader to hang
> with two MBRs, but should this be expected?  If the loader did not get
> "confused", any insight to what may have actually happened, since I
> could not see any output whatsoever?
> Regards,

Hi Glen,

I recently, successfully, broke a gmirror by simply issuing gmirror 
clear on all the disks, three in my case. Since gmirror stores metadata 
in the last sector of the disks (according to the gmirror man page) and 
everything else is untouched, issuing a gmirror clear reverts the disks 
to single disk usage, and since the data isn't striped, the system will 
be able to read the data and mount the filesystem(s).

This is my experience. However, I have found no documentation about 
breaking a gmirror, and I may just have been lucky. The data I had was 
properly backed up, so if it hadn't worked, I could have restored it, 
and since I needed the extra space, I gave it a try.

So, like I said, this worked for me, but I can't back my clam, neither 
by documentation nor reports from others, so I would still recommend 
backing your data up before issuing gmirror clear.


Rolf Nielsen

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