Problematic upgrade from 7.2 to 8.0 with ZFS file system

[Aristedes Maniatis]

Last night we upgraded one of our servers from 7.2 to 8.0-beta1 using 
the freebsd-update tool and following the instructions on Colin's blog. 
I believe this problem will occur even if we had used a source update 
mechanism.

The usual upgrade path is to install the new kernel, reboot, then 
install world. This fails because when rebooting into the 8.0 kernel 
with 7.2 userland, ZFS is unable to properly initialise and the file 
systems are not mounted. In our case we are booting with a small UFS 
partition to load the kernel, and then mounting /usr /var, etc from ZFS 
as per these instructions [1].

The problem with ZFS userland being out of sync with the kernel has been 
seen before [2] [3]. However now with lots of people running ZFS and 
starting to upgrade to 8.0, I think this will bite many more.

The workaround is to drop into single user mode, mount all the zfs 
partitions, and do the userland install.

mount -t zfs /usr
mount -t zfs /var
mount -o rw tank/root /

Those mount commands are slightly non-obvious and took a little guessing 
to get right. You can't use the 'zfs mount' command since it is broken 
at this point in time.

The other solution is to install userland BEFORE you reboot into the new 
kernel, although that may cause its own set of problems. Whatever the 
final solution, this needs to be clearly documented and ideally 
freebsd-update needs to detect the problem and advise the user about 
what to do.


Ari Maniatis




[1] http://www.ish.com.au/solutions/articles/freebsdzfs
[2] http://www.nabble.com/Re%3A-ZFS-MFC-heads-up-p23651130.html
[3] 
http://www.nabble.com/zfs-version-13-kernel-and-zfs-version-6-userland-tool--td20650216.html


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