The logfile stop upgrade after a vim write

[lz ma]

  1.   pg_ctl -D data -l  MyLog
  2.   vim MyLog : add some words, save and exit
  3.  after vim operation, MyLog will never upgrade except restart server

I know it caused by file descripter only open once at the start by postgres, 
and vim operation rename the file to MyLog~, so postgres can't upgrade