Copying mirrored partitions - will this work?

Mike Clarke jmc-freebsd2 at
Sun Mar 21 10:09:22 UTC 2010

On Sunday 21 March 2010, Modulok wrote:

> On 3/20/10, Mike Clarke <jmc-freebsd2 at> wrote:


> > I have two 500GB disks, /dev/ad4 and /dev/ad8, each partitioned
> > into 4 slices of 88, 88, 42 and 259GB. My system is installed on
> > the first slices (ad4s1 and ad8s1) which are mirrored as
> > /dev/mirror/gm0. The second slices (ad4s2 and ad8s2) are currently
> > unused. My thoughts are to temporarily add ad4s2 into gm0 with
> > "gmirror insert gm0 ad4s2" and wait for the mirror to synchronise.
> > I should then be able to remove the temporary addition with
> > "gmirror remove gm0 /dev/ad4s2" at which point ad4s2 should be a
> > duplicate of the original system and I can then go ahead and create
> > a new mirror with "gmirror label -b load gm1 ad4s2" and "gmirror
> > insert gm1 ad8s2". After editing /etc/fstab in the new mirror to
> > use gm1 instead of gm0 I should then be able to boot into the
> > system on slice 2 and upgrade it to STABLE while still keeping my
> > original system to fall back to if required.

> How valuable is your data?

In financial terms not very, but still valuable enough to not want to 
lose it.

> I recommend you make an offline backup.

Yes, I take regular backups but regard them as the "emergency parachute" 
and prefer to not put myself in a position where I'm doing something 
risky and the backup files are the only protection, so I'll be making 
additional backups anyway.

> There's a lot of steps in 
> your procedure which introduce room for error.

Yes, it's a bit of unknown territory for me but with 6 partitions on the 
slice it does require fewer potentially dangerous manual steps (like 
newfs or restore to the wrong device) so looks like an interesting 

> You could perhaps 
> disconnect one of the hard drive's data cable (same thing). Also,
> make a backup copy of your geom meta data somewhere.

That's a possibility to consider but would result in additional changes 
to the mirror configuration, something I'd prefer to keep to a minimum.

Mike Clarke

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