[Hadoop Wiki] Update of "HadoopIsNot" by SteveLoughran

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The "HadoopIsNot" page has been changed by SteveLoughran:
https://wiki.apache.org/hadoop/HadoopIsNot?action=diff&rev1=9&rev2=10

Comment:
clarify the FS a bit more

   * What the various log files your machine generates, and what they mean
   * How to set up native filesystems and mount them
  
- This is important. If you don't know these, you are out of your depth and should not start installing Hadoop until you have the basics of a couple of linux systems up and running, letting you ssh in to each of them without entering a password, know each other's hostname and such like. The Hadoop installation documents all assume you can do these things, and aren't going to bother explaining about them.
+ This is important. If you don't know these, you are out of your depth and should not start installing Hadoop until you have the basics of a couple of Linux systems up and running, letting you ssh in to each of them without entering a password, know each other's hostname and such like. The Hadoop installation documents all assume you can do these things, and aren't going to bother explaining about them.
  
  
- == HDFS is not a POSIX filesystem ==
+ == HDFS is not a complete POSIX filesystem ==
  
- The Posix filesystem model has files that can appended too, seek() calls made, files locked.You cannot seamlessly map code that assumes that all filesystems are Posix-compatible to HDFS.
+ The Posix filesystem model has expectations of what a filesystem can do that HDFS cannot. A key one is that you can only add data to the end of the file, not seek to the middle and write things. You cannot seamlessly map code that assumes that all filesystems are Posix-compatible to HDFS.