PartitionedFile is the persistent file type of sort-merge based blocking shuffle. Each
PartitionedFile contains two physical files: one is the data file and the other is the
index file. Both the data file and the index file have multiple regions. Data belonging to the
same subpartition are stored together in each data region and the corresponding index region
contains index entries of all subpartitions. Each index entry is a (long, integer) value tuple of
which the long value represents the file offset of the target subpartition and the integer value
is the number of buffers.