How can I use LVM snapshots in Ubuntu?

Note: all commands should be done from sudo

First, reduce size of your root volume to make some room for a snapshot volume

• Boot from Live CD
• fdisk -l to see name of your device.

  Device       Start       End   Sectors   Size Type
  /dev/sda1     2048   1050623   1048576   512M EFI System
  /dev/sda2  1050624   2050047    999424   488M Linux filesystem
  /dev/sda3  2050048 500117503 498067456 237.5G Linux filesystem
  

  /dev/sda3 in my case
• cryptsetup luksOpen /dev/sda3 crypt1 to decrypt your volume
• vgscan --mknodes to find all volume groups

  Reading all physical volumes. This may take a while...
  Found volume group "ubuntu-vg" using metadata type lvm2
  

• vgchange -ay to activate all your volume groups

  2 logical volume(s) in volume group "ubuntu-vg" now active
  

• lvreduce -r -L -20G /dev/ubuntu-vg/root to reduce size of your root volume by 20 Gb. This may take a while (it took ~4 min for me).
  That space will later be used to create LVM snapshots.
• vgs to see that you actually got 20G of free space

  VG           #PV   #LV   #SN   Attr     VSize     VFree
  ubuntu-vg     1      2     0   wz--n-   230.44g   20.01g
  

• reboot and remove Live CD

Create a snapshot (i.e., activate "experimentation" mode)

Now whenever you want to do something risky with your system, run the following command

• lvcreate -s -n snap -L 20G /dev/ubuntu-vg/root
  Note: /dev/ubuntu-vg/root is a logical volume that you want to have a snapshot of
• lvs to see that the volume has been created

  LV     VG            Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  root   ubuntu-vg     owi-aos--- 201.59g                                                    
  snap   ubuntu-vg     swi-a-s---  20.00g      root   0.00                                   
  swap_1 ubuntu-vg     -wi-ao----  15.90g
  

  The snapshot is active from the moment you run lvcreate. You can now start experimenting with new things.

Scenario #1: You want to revert changes

• lvconvert --merge /dev/mapper/ubuntu--vg-snap

  Logical volume ubuntu-vg/root contains a filesystem in use.
  Can't merge over open origin volume.
  Merging of snapshot ubuntu-vg/snap will occur on next activation of ubuntu-vg/root.
  

• reboot

Scenario #2: You want to persist changes

• lvremove /dev/mapper/ubuntu--vg-snap

In Conclusion

You basically have to run lvcreate to start the process and then either run lvconvert --merge or lvremove to end it.

Keep in mind that when the snapshot is active, due to Copy-On-Write strategy all the changes are saved to those 20Gb of space (or whatever value you set). One way of checking the remaining capacity is to look at Data% column of lvs command.