Chapter 8. LVM 2 Boot Time Scripts
For initrds, you should have:
vgchange -ay --ignorelockingfailure
in the linuxrc to get the root LV activated before the root
volume is accessed.
Most distros seem to have this setup in their mkinitrd scripts
now, and they also tend to have them in rc.sysinit or
equivilant, so all volumes get activated on bootup.