--thin1, --snap1 {natural}
  The numeric identifier for the first thin volume to diff.

dmsetup message /dev/mapper/volg-volg--thinpool-tpool 0 reserve_metadata_snap

# Works, but what do 1 and 2 refer to?
thin_delta --verbose -m --snap1 1 --snap2 2 /dev/mapper/volg-volg--thinpool_tmeta

# Looks nice but doesn't work
thin_delta  -m --snap1 volg/thin_volume_snap2 --snap2 volg/thin_volume_snap3 /dev/mapper/volg-volg--thinpool_tmeta

dmsetup message /dev/mapper/volg-volg--thinpool-tpool 0 release_metadata_snap

# lvs -a
  LV                    VG   Attr       LSize  Pool          Origin      Data%  Meta%  Move Log Cpy%Sync Convert
  [lvol0_pmspare]       volg ewi------- 12.00m
  thin_volume           volg Vwi-a-tz--  6.00g volg-thinpool             31.96
  thin_volume_snap1     volg Vwi---tz-k  6.00g volg-thinpool thin_volume
  thin_volume_snap2     volg Vwi---tz-k  6.00g volg-thinpool thin_volume
  thin_volume_snap3     volg Vwi---tz-k  6.00g volg-thinpool thin_volume
  volg-thinpool         volg twi-aotz--  9.80g                           20.51  9.41
  [volg-thinpool_tdata] volg Twi-ao----  9.80g
  [volg-thinpool_tmeta] volg ewi-ao---- 12.00m