> [!NOTE] ![[assets/wl_lwlock_oldsnapshottimemapping.png|256]] > > | | | > | :--- | :--- | > | **Wait Category** | LWLock | > | **Mechanism** | `OldSnapshotTimeMappingLock` | > | **Operations** | [[Operations/Table/SeqScan]], [[Operations/Tuple/ModifyTable]] | > | **Resources** | [[Resources/RAM|Memory]], [[Resources/CPU|CPU]] | > > Contention on the **Snapshot Durability Layer**. The "Old Snapshot Threshold" feature (governed by `old_snapshot_threshold`) allows the database to reclaim space from dead tuples even if a long-running transaction is still active, by allowing that transaction to eventually fail with a "snapshot too old" error.