The DistCp -update option is used to copy files from a source that do not exist at the target, or that have different contents. The DistCp -overwrite option overwrites target files even if they exist at the source, or if they have the same contents.

The -update and -overwrite options warrant further discussion, since their handling of source-paths varies from the defaults in a very subtle manner.

Consider a copy from /source/first/ and /source/second/ to /target/, where the source paths have the following contents:

hdfs://nn1:8020/source/first/1
hdfs://nn1:8020/source/first/2
hdfs://nn1:8020/source/second/10
hdfs://nn1:8020/source/second/20

When DistCp is invoked without -update or -overwrite, the DistCp defaults would create directories first/ and second/, under /target. Thus:

distcp hdfs://nn1:8020/source/first hdfs://nn1:8020/source/second hdfs://nn2:8020/target

would yield the following contents in /target:

hdfs://nn2:8020/target/first/1
hdfs://nn2:8020/target/first/2
hdfs://nn2:8020/target/second/10
hdfs://nn2:8020/target/second/20

When either -update or -overwrite is specified, the contents of the source directories are copied to the target, and not the source directories themselves. Thus:

distcp -update hdfs://nn1:8020/source/first hdfs://nn1:8020/source/second hdfs://nn2:8020/target

would yield the following contents in /target:

hdfs://nn2:8020/target/1
hdfs://nn2:8020/target/2
hdfs://nn2:8020/target/10
hdfs://nn2:8020/target/20

By extension, if both source folders contained a file with the same name ("0", for example), then both sources would map an entry to /target/0 at the destination. Rather than permit this conflict, DistCp will abort.

Now, consider the following copy operation:

distcp hdfs://nn1:8020/source/first hdfs://nn1:8020/source/second hdfs://nn2:8020/target

With sources/sizes:

hdfs://nn1:8020/source/first/1 32
hdfs://nn1:8020/source/first/2 32
hdfs://nn1:8020/source/second/10 64
hdfs://nn1:8020/source/second/20 32

And destination/sizes:

hdfs://nn2:8020/target/1 32
hdfs://nn2:8020/target/10 32
hdfs://nn2:8020/target/20 64

Will effect:

hdfs://nn2:8020/target/1 32
hdfs://nn2:8020/target/2 32
hdfs://nn2:8020/target/10 64
hdfs://nn2:8020/target/20 32

1 is skipped because the file-length and contents match. 2 is copied because it doesn’t exist at the target. 10 and 20 are overwritten because the contents don’t match the source.

If the -update option is used, 1 is overwritten as well.