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don't forget mkinitrd

Most Linux system-administrators are well aware of the benefits of LVM. With online resizing of filesystems and migration of data from one disk to another, it's fantastic. But don't assume that your system will do everything for you.
If your system has one disk, that you used as a physical volume (PV) in a volume group (VG), where your root partition is stored as a logical volume (LV), you can easily add a new disk. Add the disk physically, boot up, use pvcreate and vgextend to include the new disk in the existing volume group. Just don't reboot, at least, not yet !

Your system requires a manual rebuild of its initial ram disk
/sbin/mkinitrd -f /boot/initrd-2.6.18-92.1.10.el5.img 2.6.18-92.1.10.el5

If you forget to do this, your system will not boot, because it won't find all the components necessary to activate the VG it needs to access the root filesystem. Symptoms: kernel loads, initrd loads, root filesystem can't be mounted because the volume group doesn't exist.
If it happened already (and that's why google sent you to this blog entry), don't despair, it's an easy thing to fix with the "linux rescue" option of your RHEL/CentOS installation CD or DVD.

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