@@ -6847,6 +6847,18 @@ LogCheckpointEnd(bool restartpoint)
68476847 * Note: flags contains other bits, of interest here only for logging purposes.
68486848 * In particular note that this routine is synchronous and does not pay
68496849 * attention to CHECKPOINT_WAIT.
6850+ *
6851+ * If !shutdown then we are writing an online checkpoint. This is a very special
6852+ * kind of operation and WAL record because the checkpoint action occurs over
6853+ * a period of time yet logically occurs at just a single LSN. The logical
6854+ * position of the WAL record (redo ptr) is the same or earlier than the
6855+ * physical position. When we replay WAL we locate the checkpoint via its
6856+ * physical position then read the redo ptr and actually start replay at the
6857+ * earlier logical position. Note that we don't write *anything* to WAL at
6858+ * the logical position, so that location could be any other kind of WAL record.
6859+ * All of this mechanism allows us to continue working while we checkpoint.
6860+ * As a result, timing of actions is critical here and be careful to note that
6861+ * this function will likely take minutes to execute on a busy system.
68506862 */
68516863void
68526864CreateCheckPoint (int flags )