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From: Chris St Denis <chris@smartt.com>
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Cc: freebsd-questions@freebsd.org
Subject: Re: A gmirror question.
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Stefan Moro wrote:
> Hi!
>
> I've got a question regarding the way that gmirror identifies what
> "components" (if that is the right term) are included in a mirror.
>
> I recently created a mirror over two disks, ad2 and ad4. After some
> bios changes (activating PATA) these devices were changed to ad6 and
> ad10 respectively.
> The "magic" thing (which probably isn't so magic =) ) here is that
> after reboot, gmirror still managed to identify the mirror, now using
> ad6 and ad10.
>
> If I have understood things correctly, gmirror stores all relevant
> information about the mirror in the last sector on the provider.
>
> So I guess one possible solution is that gmirror ,during startup,
> scans all disks/slices for this magic sector and then "starts" the
> mirror.
> But if this is the case, how does gmirror know that it should use (in
> my example above) ad10 and not ad4 as the additional disk in the
> mirror.
> Or does gmirror use something else than the /dev entries to address disks??
>
> I'm just curious how gmirror does this.
>
>
> BR Stefan
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>   

I don't know the technical details of the internal working of gmirror, 
but it likely uses the volume serial number or other such identifier.