log file can't rotate

[superhjw]

found that the log file can't roate to another file, ex: system.log, when maxsize will not to change to system.log.1.
code:
lvl := log4go.INFO
switch CFG.UString("log.level", "INFO") {
case "DEBUG":
lvl = log4go.DEBUG
case "INFO":
lvl = log4go.INFO
case "WARNING":
lvl = log4go.WARNING
case "ERROR":
lvl = log4go.ERROR
case "CRITICAL":
lvl = log4go.CRITICAL
default:
lvl = log4go.INFO
}
LOG.AddFilter("stdout", lvl, log4go.NewConsoleLogWriter())
logWriter := log4go.NewFileLogWriter("system.log", true, true)
logWriter.SetRotateSize(10)
logWriter.SetRotateLines(1000)
LOG.AddFilter("log", lvl, logWriter)