Postgres log files not rotating
Try the following log parameters. After modify them, remember to reload the configuration file ($PGDATA/postgresql.conf). 

# These are only used if logging_collector is on:
log_directory = '/var/applog/pg_log/1921'               # directory where log files are written,
                                        # can be absolute or relative to PGDATA
log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log' # log file name pattern,
                                        # can include strftime() escapes
log_file_mode = 0600                    # creation mode for log files,
                                        # begin with 0 to use octal notation
log_truncate_on_rotation = on           # If on, an existing log file with the
                                        # same name as the new log file will be
                                        # truncated rather than appended to.
                                        # But such truncation only occurs on
                                        # time-driven rotation, not on restarts
                                        # or size-driven rotation.  Default is
                                        # off, meaning append to existing files
                                        # in all cases.
log_rotation_age = 1d                  # Automatic rotation of logfiles will
                                        # happen after that time.  0 disables.
log_rotation_size = 10MB               # Automatic rotation of logfiles will