My laptop has been waiting for ages to get the KDE monolithic to KDE split update done. It's just so annoying needing to unmerge the system while you're upgrading. So today I decided to try using aufs to create a chrooted copy-on-write version of the system.

sudo mount -t aufs -o dirs=installroot:/=ro none fakeroot
sudo chroot /mnt/secondary/fakeroot/ /bin/bash

From within the chroot it's just a matter of unmerging the kde monolithic ebuilds and then merging the particular kde packages you want. You can keep using all of kde while the chroot is building the whole planned update, no problem if you stop in the middle, you haven't touched the kde outside the chroot (the one you actually use).

When it's all done I'll merge the portage "packages" directory into the main system directory and I'll be able to do a binary-only update that will only leave me desktop-less for a few minutes. That's the plan, anyway...