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How much space does a btrfs snapshot actually exclusively allocate? One simply has to run a btrfs fi du -s backup-* in order to see which space is shared between the snapshots and which is exclusively used by the snapshot listed.

     Total   Exclusive  Set shared  Filename
   2.80TiB     5.79GiB     2.80TiB  backup-2023-08-20-01-00-01
   2.80TiB     5.51GiB     2.80TiB  backup-2023-08-21-01-00-01
   2.81TiB       0.00B     2.81TiB  backup-2023-08-22-01-00-01

In the example above, backup-2023-08-22-01-00-01 is the latest snapshot. Since, in this case, this snapshot's source hasn't changed the snapshot doesn't allocate any space exclusively.