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Date: Wed, 08 Jun 2011 19:40:03 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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To: Jeremy Chadwick <freebsd@jdc.parodius.com>
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Cc: freebsd-stable@FreeBSD.org
Subject: Re: [SOLVED] Re: labelling root file system (RELENG_8)
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on 08/06/2011 19:26 Jeremy Chadwick said the following:
> I have the exact same question except not with regards to labels but
> toggling TRIM capability on the root filesystem.
> 
> - Start system
> - At loader, boot single-user (option 4)
> - At prompt choose /bin/sh
> - mount -a

I think that this is a culprit.

> - tunefs -t enable /dev/ada0s1a --- fails

Shouldn't you have / mounted r/o here?
BTW, AFAIR, *re*-mounting root read-only won't help; it needs to have never been
mounted r/w.

> - sysctl kern.geom.debugflags=16
> - tunefs -t enable /dev/ada0s1a --- works
> - tunefs -p /dev/ada0s1a -- shows TRIM enabled
> - reboot

I think that at this step your superblock on disk gets re-written with its copy in
memory which has never been updated.  But not sure.

> - Boot into single-user, multi-user, whatever
> - tunefs -p /dev/ada0s1a -- shows TRIM disabled

-- 
Andriy Gapon