code: public void flush(LogSeqNum lsn) {
  		logMgrLock.lock();
  		try {
  			if (lsn.compareTo(lastFlushedLsn) >= 0)
  				flush();
  		} finally {
  			logMgrLock.unlock();
  		}
  	}
docs: Ensures that the log records corresponding to the specified LSN has been
  written to disk. All earlier log records will also be written to disk.
  
  @param lsn
  the LSN of a log record