FileSystem
FileSystem is an abstract base class that provides some common methods for LocalFileSystem and DistributedFileSystem. It maintains all used file systems via a HashMap: name -> filesystem, whose key is either “Local” or “Host:Port” (identifying a NameNode).
It inherits from the Configured class and can load some basic parameters via configuration, saved in Configuration.
To improve reliability, it generates a checksum for each file and saves it in a hidden file with the extension .*.crc.
