# Third Step: `GC` All Keys

+ `v4.0.x`:
  + For all `leaders` on the `TiKV` node: 
    + For all keys in the region:
      + Scan the key's versions and delete those obsoleted ones
  + Scanning and deleting is performed on `Raft` layer
  + Most `regions` without many versions to delete can be skipped
    + Based on certain metadata in each `SST` file (oldest/latest version, number of versions, number of deleted versions, ...), namely **`MvccProperties`**


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