# lvmmerge - dracut module ## Preparation - ensure that the lvm thin pool is big enough - backup any (most likely /boot and /boot/efi) device with: ``` # mkdir /restoredev # dev=<device>; umount $dev; dd if="$dev" of=/restoredev/$(systemd-escape -p "$dev"); mount $dev ``` - backup the MBR ``` # dev=<device>; dd if="$dev" of=/restoredev/$(systemd-escape -p "$dev") bs=446 count=1 # ls -l /dev/disk/by-path/virtio-pci-0000\:00\:07.0 lrwxrwxrwx. 1 root root 9 Jul 24 04:27 /dev/disk/by-path/virtio-pci-0000:00:07.0 -> ../../vda ``` - backup some partitions ``` # dev=/dev/disk/by-path/virtio-pci-0000:00:07.0 # dd if="$dev" of=/restoredev/$(systemd-escape -p "$dev") bs=446 count=1 # umount /boot/efi # dev=/dev/disk/by-partuuid/687177a8-86b3-4e37-a328-91d20db9563c # dd if="$dev" of=/restoredev/$(systemd-escape -p "$dev") # umount /boot # dev=/dev/disk/by-partuuid/4fdf99e9-4f28-4207-a26f-c76546824eaf # dd if="$dev" of=/restoredev/$(systemd-escape -p "$dev") ``` Final /restoredev ``` # ls -al /restoredev/ total 1253380 drwx------. 2 root root 250 Jul 24 04:38 . dr-xr-xr-x. 18 root root 242 Jul 24 04:32 .. -rw-------. 1 root root 209715200 Jul 24 04:34 dev-disk-by\x2dpartuuid-4fdf99e9\x2d4f28\x2d4207\x2da26f\x2dc76546824eaf -rw-------. 1 root root 1073741824 Jul 24 04:34 dev-disk-by\x2dpartuuid-687177a8\x2d86b3\x2d4e37\x2da328\x2d91d20db9563c -rw-------. 1 root root 446 Jul 24 04:38 dev-disk-by\x2dpath-virtio\x2dpci\x2d0000:00:07.0 ``` - make a thin snapshot ``` # lvm lvcreate -pr -s rhel/root --name reset ``` - mark the snapshot with a tag ``` # lvm lvchange --addtag reset rhel/reset ``` - remove /restoredev ``` # rm -fr /restoredev ``` ## Operation If a boot entry with ```rd.lvm.mergetags=<tag>``` is selected and there are LVs with ```<tag>``` dracut will - make a copy of the snapshot - merge it back to the original - rename the copy back to the name of the snapshot - if /restordev appears in the root, then it will restore the images found in that directory. This can be used to restore /boot and /boot/efi and the MBR of the boot device