Similar to the following bug:

https://bugzilla.redhat.com/show_bug.cgi?id=719398

When using systemd, the kernel boot messages are not dumped into
/var/log/dmesg.  With the old sysv-style init, /etc/rc.d/rc.sysinit included:

# Now that we have all of our basic modules loaded and the kernel going,
# let's dump the syslog ring somewhere so we can find it later
[ -f /var/log/dmesg ] && mv -f /var/log/dmesg /var/log/dmesg.old
dmesg -s 131072 > /var/log/dmesg

This is very useful to have; a single file with the boot messages that is not
logrotated away.