晋太元中，武陵人捕鱼为业。缘溪行，忘路之远近。忽逢桃花林，夹岸数百步，中无杂树，芳草鲜美，落英缤纷。渔人甚异之，复前行，欲穷其林。 　　林尽水源，便得一山，山有小口，仿佛若有光。便舍船，从口入。初极狭，才通人。复行数十步，豁然开朗。土地平旷，屋舍俨然，有良田、美池、桑竹之属。阡陌交通，鸡犬相闻。其中往来种作，男女衣着，悉如外人。黄发垂髫，并怡然自乐。 　　见渔人，乃大惊，问所从来。具答之。便要还家，设酒杀鸡作食。村中闻有此人，咸来问讯。自云先世避秦时乱，率妻子邑人来此绝境，不复出焉，遂与外人间隔。问今是何世，乃不知有汉，无论魏晋。此人一一为具言所闻，皆叹惋。余人各复延至其家，皆出酒食。停数日，辞去。此中人语云：“不足为外人道也。”(间隔 一作：隔绝) 　　既出，得其船，便扶向路，处处志之。及郡下，诣太守，说如此。太守即遣人随其往，寻向所志，遂迷，不复得路。 　　南阳刘子骥，高尚士也，闻之，欣然规往。未果，寻病终。后遂无问津者。 .
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# 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

haha - 2025