Udev/Upgrade Guide

udev 197
The following special attention is required:


 * CONFIG_DEVTMPFS=y must be set in the kernel, it is wise to set CONFIG_DEVTMPFS_MOUNT=y as well to avoid early boot hangs.


 * If there is a /dev line in /etc/fstab, it must have the devtmpfs filesystem type or it will conflict with the above options.


 * Remove udev-postmount from any runlevels, it is not necessary.


 * If you are using /etc/udev/rules.d/70-persistent-net.rules, read [bug #453494]; this won't be a problem if you use [Predictable Network Interface Names].


 * Since this version, support for older kernels than 2.6.39 is dropped, if you still use such a kernel and can't upgrade consider to use the eudev fork.

Upgrading to the latest gentoo-sources version is also recommended.

For more details, read the full news article /usr/portage/metadata/news/2013-01-23-udev-upgrade/2013-01-23-udev-upgrade.en.txt and the post install output of emerging udev.

udev 181
Since this version udev depends on files in. If you have on a separate partition, you must boot your system with an initramfs which pre-mounts.

There are many tools to create such a initramfs:
 * genkernel
 * Dracut
 * See the initramfs article for more solutions

For more details, see http://freedesktop.org/wiki/Software/systemd/separate-usr-is-broken and the post install output of emerging udev.