why last_sent_lsn can be lower the last_received lsn ?
and the last harden time is much later than the last received time ??
and the last redone lsn is less than last_received lsn
and what about the end_of_log_lsn and last_commit_lsn ? how is it related to last_harden_lsn??
what's the meaning of low_water_mark_for_ghosts??