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
yields 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
Effects:
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 does not exist at the target.
10 and 20 are overwritten because the contents do not
match the source.
If the -update option is used, 1 is overwritten as well.
