Update to section 19.4 on disk mirroring

[Tom Rhodes]

On Mon, 25 Aug 2008 23:39:46 -0400 (EDT)
John R Levine <johnl at taugh.com> wrote:

> I just added an extra disk to a BSD server to mirror the existing one, and 
> looked at section 19.4 on GEOM mirroring.  While the advice it gives on 
> setting up bootable gmirror disks isn't wrong, it's seriously suboptimal 
> since it involves a needless full copy of a disk, which takes hours rather 
> than five minutes if you do it the easy way.
> 
> I dunno what the etiquette is for updating the handbook, but I'd be happy 
> to redo this part to say, roughly:
> 
> It's straightforward to convert any bootable FreeBSD disk to a gmirror 
> mirror if you have a second disk of the same size.  Call the existing disk 
> ad0 and the new second disk ad2.
> 
> First, use fdisk to set the slices on the second disk identically to the 
> first disk, using fdisk -p to dump the slice config of ad0 and fdisk -f to 
> apply that config to ad2.  Put geom_mirror_load="YES" into 
> /boot/loader.conf to ensure that gmirror is loaded at boot time, or build 
> and install a new kernel with options GEOM_MIRROR.
> 
> You can't do GEOM work when any of the file systems are mounted, so 
> reboot from a live FS CD.  Create the mirror, initially with the single 
> existing disk:
> 
>   # gmirror label -v gm0 ad0
> 
> You should see a kernel message confirming the creation of the mirror, and 
> a listing /dev/mirror should show an entry corresponding to each partition 
> on the disk, e.g., for /dev/ad0s1a now there's /dev/mirror/gm0s1a
> 
> You need to change fstab to refer to the mirrored file systems.  Mount 
> your root fs on /mnt so you can edit it:
> 
>   # mount /dev/mirror/gm0s1a /mnt

I like to skip this step altogether ... but you're right, the
section needs re-written.  I have something, it's not marked up
yet.  Perhaps I'll work on that this week.

-- 
Tom Rhodes