Docs for SERVER-33349: Add command to get stable checkpoint timestamp

Scope of changes:

We've added an optional field to replSetGetStatus called "lastStableCheckpointTimestamp". On storage engines that do not support recover to stable timestamp (all but WT for now), this field will be missing. Otherwise it will be a Timestamp at or before which the current persisted checkpoint was taken. This may be lagged, but if it exists then that timestamp is guaranteed to be persisted in a stable checkpoint to disk.

• replSetGetStatus
  • add new field
  • update sample output
• 4.0.txt

Impact to other docs outside of this product:

MVP:

Resources:

Engineering Ticket Description:

Now that the storage engine is maintaining this information, we need to expose it to our tests and users like automation.