InputFormats and MapReduce Components
The InputFormats and MapReduce components are responsible for the actual copying of files and directories from the source to the destination path. The listing file created during copy-listing generation is consumed at this point, when the copy is carried out. The classes of interest here include:
UniformSizeInputFormat: This implementation of org.apache.hadoop.mapreduce.InputFormat provides equivalence with Legacy DistCp in balancing load across maps. The aim of the UniformSizeInputFormat is to make each map copy roughly the same number of bytes. Therefore, the listing file is split into groups of paths, such that the sum of file sizes in each InputSplit is nearly equal to every other map. The splitting is not always perfect, but its trivial implementation keeps the setup time low.
DynamicInputFormat and DynamicRecordReader: The DynamicInputFormat implements org.apache.hadoop.mapreduce.InputFormat, and is new to DistCp. The listing file is split into several “chunk files”, the exact number of chunk files being a multiple of the number of maps requested for in the Hadoop Job. Each map task is “assigned” one of the chunk files (by renaming the chunk to the task’s id), before the Job is launched. Paths are read from each chunk using the DynamicRecordReader, and processed in the CopyMapper. After all of the paths in a chunk are processed, the current chunk is deleted and a new chunk is acquired. The process continues until no more chunks are available. This “dynamic” approach allows faster map tasks to consume more paths than slower ones, thus speeding up the DistCp job overall.
CopyMapper: This class implements the physical file copy. The input paths are checked against the input options (specified in the job configuration), to determine whether a file needs to be copied. A file will be copied only if at least one of the following is true:
A file with the same name does not exist at target.
A file with the same name exists at target, but has a different file size.
A file with the same name exists at target, but has a different checksum, and - skipcrccheck is not mentioned.
A file with the same name exists at target, but -overwrite is specified.
A file with the same name exists at target, but differs in block-size (and block-size needs to be preserved).
CopyCommitter: This class is responsible for the commit phase of the DistCp job, including:
Preservation of directory permissions (if specified in the options)
Clean up of temporary files, work directories, etc.