While working on an automated install script for an embedded board, I hit an issue with the logical volumes never showing up in /dev/mapper, and in turn unable to be mounted. This left me in the dracut emergency shell (after about three minutes), with little to go on beyond the following error:

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[187.508531] dracut Warning: Could not boot.
[187.510560] dracut Warning: /dev/disk/by-uuid/5681-902D does not exist
[187.512534] dracut Warning: /dev/mapper/system-root does not exit
[187.513990] dracut Warning: /dev/system/root does not exist

After booting into a live CD I checked to make sure the volume group showed up under pvscan like so:

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# pvscan
  PV /dev/mmcblk0p3   VG system     lvm2 [19.50 GiB / 0    free]
  Total: 1 [19.50 GiB] / in use: 1 [19.50 GiB] / in no VG: 0 [0   ]

And the logical volumes were also showing up with lvscan:

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# lvscan
  inactive          '/dev/system/root' [18.45 GiB] inherit
  inactive          '/dev/system/swap' [1.05 GiB] inherit

Notice that they are both marked inactive? That's our issue. To fix it we can mark all logical volumes under our volume group as active (replace "system" with your volume group name):

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$ vgchange --activate y system

This didn't fix the LVM volumes showing up at boot but it did allow me to get back into my root filesystem as a chroot so I could investigate the issue which I've finished documenting in another post.