Updating a ZFS on Root installation in FreeBSD 8
Ever since ZFS became production ready on FreeBSD 8 (and backported to 7), I’ve been itching to switch to using it and getting rid of UFS. For one, UFS is OLD. It was first used in 4.2BSD if that gives any indication. The most salient problem with UFS is the lack of real journaling. True, you have the soft-updates feature, but soft-updates are an alternative to journaling. This may not be a problem for users with older hardware and smaller hard disks, but with today’s multi-terabyte drives, a power failure or system crash can lead to painfully long waits as fsck verifies the consistency of your file system.
To that end, I followed this handy guide to installing a FreeBSD system on a pure ZFS-only setup. Note that you’ll end up with a system with no traces of UFS whatsoever, meaning you have to use the ZFS bootloader and can’t dual boot anymore. If that’s not to your liking, have a look at the index of ZFS on root guides. If you need to dual boot, you’ll want to follow one of the MBR guides. For my needs, if I find myself needing to boot to another operating system, I simply hit F12 when my system is performing a POST test and choose a different drive to boot to. It’s simple and separates your operating systems so they don’t affect one another. Another alternative is to use virtualization if your needs aren’t too demanding.
One thing none of the ZFS on root guides show you is also one of the most important: how to update and rebuild your system! If you’re a developer you probably already know how to do this, but for the rest of us, it’s important to know how to properly update your system when needed.
To begin, you’ll want to sync your source tree. Instructions for that are located in the handbook, but it really boils down to using csup to synchronize source from a cvsup server to your local machine. If you do anything that requires building a kernel module, you’re probably familiar with this. With your source tree synchronized to your desired version of FreeBSD, you can follow the the handbook guide to rebuilding world.
Once you have installed the kernel, reboot into single user mode as the guide tells you. Issue the “mount” command. You’ll notice that no filesystems are mounted except for devfs and the root ZFS filesystem you marked as legacy. The handbook tells you to use “mount -a -t ufs” but obviously this won’t work since we have banished UFS from our FreeBSD box. To mount all the ZFS mountpoints you have, simply use “zfs mount -a”. This should mount everything except for / (root) as read-write. To enable a writable file system root, use “mount -uw /” and then “mount -a” to mount anything else. The “-uw” option unsets any mount options and then sets the write bit so you can write to that mount point. At this point you should be able to use mergmaster and installworld to finish rebuilding your system. One important task you must do before rebooting into your updated system is to install updated boot code. This is done with the following command after installing the new world: “gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 disk” where disk is the name of the drive you installed to (usually ad0). With this, you should have an updated system with ZFS as your file system!
Enjoy!