If you are having the following error on your Gentoo system:
Can't open the database file '/var/cache/eix/portage.eix' for writing (mode = 'wb')
Don’t waste your time, simply the /var/cache/eix directory is not present and/or writeable by the eix/portage use
mkdir -p /var/cache/eix chmod +w /var/cache/eix*
Basic story is that eix will drop privileges to portage user when ran as root.