How to mirror the FreeBSD OS on two disks

Joseph Lenox lordofhyphens at
Thu Jul 12 14:21:59 UTC 2012

On 07/12/2012 05:47 AM, Mike Clarke wrote:
> On Wednesday 11 July 2012 16:20:41 Joseph Lenox wrote:
>> What about a ZFS root? Just make sure both disks are in the BIOS/EFT
>> boot order.
>> Something else we noticed on our site is that backup of a system
>> snapshot can be quickly restored using just a live CD (do up to step 5,
>> then replace steps 6-7 with a zfs receive of the desired snapshot).
> Since the system is to be restored from the snapshot then I suppose most of
> steps 8 to 12 wouldn't be needed either. But what about step 5 before the
> restore:
> zpool export zroot
> zpool import -o cachefile=/var/tmp/zpool.cache zroot
> And then step 10 after running zfs receive
> cp /var/tmp/zpool.cache /mnt/boot/zfs/zpool.cache
> Are these steps needed when restoring from a snapshot?
I believe preserving the zpool cache is important, but I haven't tested 
not doing so. Logically, the zroot is still new, and the restore from 
snapshot would still populate the cachefile (which would default to 
writing in the live CD's /var/tmp, not the target system's /var/tmp.

Here's my suggested instructions, adapted from and my own experimentation, 
for restoring from a snapshot. It can also be used to clone a system 
configuration from one system to another (very convenient). This is for 
single-drive, just set up a mirror in the initial steps if you are going 
that route.

 1. Boot from a FreeBSD9 installation DVD or memstick and choose "Live CD".
 2. Create the necessary partitions on the disk(s) and add ZFS aware
    boot code.

        gpart create -s gpt ada0

        gpart add -b 34 -s 94 -t freebsd-boot ada0

        gpart add -t freebsd-zfs -l disk0 ada0

        gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada0

 3. Align the Disks for 4K and create the pool.

        gnop create -S 4096 /dev/gpt/disk0

        zpool create -o altroot=/mnt -o cachefile=/var/tmp/zpool.cache zroot /dev/gpt/disk0.nop

        zpool export zroot

        gnop destroy /dev/gpt/disk0.nop

        zpool import -o altroot=/mnt -o cachefile=/var/tmp/zpool.cache zroot

 4. Set the bootfs property on zroot.

        zpool set bootfs=zroot zroot

 5. Mount the memory stick containing the snapshot. Most memory sticks
    are formatted fat32 or ntfs, and the LiveCD will at least read ntfs.

        mount -t ntfs /da0s1 /media/

      * This assumes that the memory stick is NTFS formatted and it ends
        up as da0 in the system.
 6. Receive snapshot.

        gunzip -dc /media/snapshot_name.gz | zfs receive -vF zroot

      * snapshot_name.gz is a placeholder for the actual name of the
        file on the media. I've assumed that the snapshot is gzip'd,
        otherwise cat the snapshot file.
 7. Copy zpool.cache (very important!!!)

        cp /var/tmp/zpool.cache /mnt/boot/zfs/zpool.cache

 8. If this is a clone of another system, edit pre-existing rc.conf and
    rc.local.conf files to suit new network configuration.
      * Specifically, the hostname and the IP need to change if the new
        system is on the network.
 9. Reboot
      * Remember to set the correct boot drive in new system BIOS.

--Joseph Lenox, BS, MS
I'm an engineer. I solve problems.

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