Using Geom to mirror root partitions?

Josef Karthauser joe at
Fri Nov 14 17:41:35 PST 2003

On Sat, Nov 15, 2003 at 12:32:10AM +0100, Poul-Henning Kamp wrote:
> In message <20031114232009.GA31237 at>, Josef Karthauser writes:
> >I've got a machine with two drives 120gb drives, which is going to a
> >colo.  If I configure it to use one drive, will I at some point be able
> >to remotely reconfigure it to say use the second drive as a mirror as
> >the physical layer, i.e. if the first drive goes bang then I can get the
> >second drive to boot via the bios and still have the machine work - or
> >am I dreaming?
> The main problem here (currently) is the bootblocks and /etc/fstab.
> We had some work on geom_mirror recently which I have not yet had
> time to look at.  If that is done right, it should be possible to
> set it up like you say.
> In order to "fool" the bootblocks, geom_mirror would have to be able
> to use a meta-data sector at the end of the partition rather than at
> the beginning.  With that in place it should "just work", since the
> device in /etc/fstab would then be the geom_mirror device and GEOM
> should be able to offer that for root mounting.

What's the best way to prepare for this?  Should I leave some
unallocated space at the beginning of the disk so that any magic geom
bits can be inserted later?

Josef Karthauser (joe at
FreeBSD (cvs meister, admin and hacker)
Physics Particle Theory (student)
================ An eclectic mix of fact and theory. =================
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