If a Cut The Crap object manager preserves Session Data then no data that existed prior to the current session will be overwritten.
The Write Once object manager uses the Write Once store and never overwrites existing data.
The standard read-write object manager can be requested to preserve session data. In which case any free'd storage is not able to be re-allocated until the start of the next session.
If previous state is retained within a session, then it is possible to grab snapshot object managers that can be used to retrieve any objects in the state when the snapshot was made.
In this way an isolated transaction snapshots the current object manager and uses this when making any external data request. This guarantees that it can only read data that was already committed at the time of isolation.
The session ends when the system is closed. When the next session starts, the historical data protected in the previous session can now be reallocated.