[PC-BSD Testing] Special FS Layout idea

[Alexander Yerenkow]

Hello there.

I just found out that ZFS now supports unionfs :)
So, I want make long standing idea real - to test graceful OS patches 
(which was possible for UFS, but I wanted ZFS).

I think that basically, system could have this layout:

/ (few gigs here)
/boot (few gigs here - since we could want new kernel testing)
/home (for user's data - no software here)
/usr (10-30g, basic system goes here codename USRA)
/over-usr ( it mounts  --- unionfs /over-usr /usr, all PBIs and ports 
and etc-configs goes here)
/var/db/pkg is symlink to /usr/var/db/pkg (will explain further)
(one extra partition, same size as /usr - codename USRB).


Testing upgrade (RFC, please feel free to comment):

1. install new kernel as kernel.new, make it nextkernel (or write it 
somewhere in loader.conf as new default kernel, if possible)
2. install (ideally, simply untar or even `dd` if possible) new system 
to USRB, edit fstab (change mount point for /usr from USRA to USRB), and 
simply reboot.
3. after reboot you have same all PBI-s, manually edited configs, 
everything else in place, and with extra bonus new operating system (new 
kernel + new base system + new installed packages)
3a. You like system - you run "make this version working", which do 
(kernel -> kernel.old, kernel.new -> kernel), changes in fstab are 
saved, and you need simply reboot.
3b. There's something went totally wrong (as always could be), you just 
revert fstab, and simply reboot to old system.

Anyone see some caveats, or maybe some advices? :)
If not, I'll start to making test environment, will toy with upgrading 9 
to 9-STABLE.