If you build a LVM mdraid5 on Ubuntu 14.04, and update the kernel you may be dropped into initramfs on reboot and be forced to manually activate the logical volumes on the volume group. This is due to a missing/incomplete udev rule for LVM which should be incorporated into initramfs.
/etc/udev/rules.d/85-lvm2.rules
# This file causes block devices with LVM signatures to be automatically
# added to their volume group.
# See udev(8) for syntax
SUBSYSTEM=="block", ACTION=="add|change", ENV{ID_TYPE}=="disk", \
RUN+="watershed sh -c '/sbin/lvm vgscan; /sbin/lvm vgchange -a y'"
Once you have added this udev rule, you should update initramfs on your system:
update-initramfs -u -k all
If you need to activate the logical volume groups from initramfs, execute the following commands to boot the system:
lvm vgscan lvm vgchange -a y exit