Copying mirrored partitions - will this work?

Mike Clarke jmc-freebsd2 at
Wed Mar 31 09:22:46 UTC 2010

On Saturday 20 March 2010, Mike Clarke wrote:

> I'm currently running 8.0-RELEASE and am considering experimenting
> with 8.0-STABLE. I'd like to preserve my existing system in case
> things go pear-shaped so I'll copy the entire system onto a spare
> slice and then use csup to upgrade the copy to STABLE. Normally I'd
> go through the steps of bsdlabel, newfs and then dump|restore to
> create the copy but I'm wondering if I can take advantage of my
> recently created gmirror to cut down the work.
> 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.
> Is this approach of moving disks from one mirror to another workable,
> or have I missed something that would lead me into deep trouble? I
> don't mind unduly if I make a mess of the second slice and have to
> start again but I don't want to lose the contents of my original
> system on slice 1.

I decided to give it a try and the process went through very smoothly. 
It was much less tedious than bsdlabel -> newfs -> dump|restore, and 
quicker too. The mirror synchronised at a bit over 100 MB/sec but dump|
restore only gave me about 10 MB/sec.

The system has now been running for a bit over a week without any 
problems with either the original or cloned slices so I'm quite 
confident that things are OK.

Mike Clarke

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