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1. Configuration - add one configuration item "log.preallocate",  parse it in KafkaConfig.scala, and transfer to KafkaServer.scala, LogConfig.scala.
2. In Log.scala 
   1.  add one function initFileSize().  if the log preallocation is enabled, the value is config.segmentSize - 2 * config.maxMessageSize,  else the value is 0.
   2.  pass the initFileSize and config.preallocate to LogSegment when need create one LogSegment.
   3.  When need roll to a new file, trim log file of active segment
3. In LogSegment.scala -
   1. add one constructor to pass the initFileSize and preallocation to FileMessageSet.     
4. In FileMessageSet.scala 
   1.  add one function trim().  Call it when close or roll a new LogSegment.
   2. Move openChannel from CoreUtil.scala and add two more parameter "initFileSize" and "preallocate".
   3. Add one constructor function with two more parameters  "initFileSize" and "preallocate".
      1. if preallocate is false, initFileSize will be ignored and will be same as other constructor function,
5. In CoreUtils.scala
   1. Move openChallel to FileMessageSet.scala

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This change is fully backward compatible.

Below are detail description for the scenarios:

1. If do clear shutdown,  the last log file will be truncated to its real size since the close() function of FileMessageSet will call trim(),
2. If crash, then when restart,  will go through the process of recover() and the last log file will be truncate to its real size, (and the position will be moved to end of the file)
3. When service start and open existing file
   1. Will run the LogSegment constructor which has NO parameter "preallocate",
   2. Then in FileMessageSet,  the "end" in FileMessageSet will be Int.MaxValue,   and then "channel.position(math.min(channel.size().toInt, end))"  will make the position be end of the file,
   3. If recover needed, the recover function will truncate file to end of valid data, and also move the position to it,

    

4. When service running and need create new log segment and new FileMessageSet
   
1. If preallocate = true
   1. the "end" in FileMessageSet will be 0,  the file size will be "initFileSize", and then "channel.position(math.min(channel.size().toInt, end))"  will make the position be 0,
2. Else if preallocate = false
   1. backward compatible, the "end" in FileMessageSet will be Int.MaxValue, the file size will be "0",  and then "channel.position(math.min(channel.size().toInt, end))"  will make the position be 0,

Rejected Alternatives