Hadoop DistCp

Hadoop DistCp is a command line, MapReduce-based tool for bulk data copying.

The hadoop distcp command can be used to copy data to and from Ozone and any Hadoop compatible file systems, such as HDFS or S3A.

Basic usage

To copy files from a source Ozone cluster directory to a destination Ozone cluster directory, use the following command:

hadoop distcp ofs://ozone1/vol1/bucket/dir1 ofs://ozone2/vol2/bucket2/dir2

You must define the service IDs for both ozone1 and ozone2 clusters in the ozone-site.xml configuration file. For example:

<property>
  <name>ozone.om.service.ids</name>
  <value>ozone1,ozone2</value>
</property>

Next, define their logical mappings. For more details, refer to OM High Availability.

Copy from HDFS to Ozone

DistCp performs a file checksum check to ensure file integrity. However, since the default checksum type of HDFS (CRC32C) differs from that of Ozone (CRC32), the file checksum check will cause the DistCp job to fail.

To prevent job failures, specify checksum options in the DistCp command to force Ozone to use the same checksum type as HDFS. For example:

hadoop distcp \
  -Ddfs.checksum.combine.mode=COMPOSITE_CRC \
  -Dozone.client.checksum.type=CRC32C \
  hdfs://ns1/tmp ofs://ozone1/vol1/bucket1/dst

Note: The parameter -Ddfs.checksum.combine.mode=COMPOSITE_CRC is not required if the HDFS cluster is running Hadoop 3.1.1 or later.

Alternatively, you can skip the file checksum check entirely:

hadoop distcp \
  -skipcrccheck \
  hdfs://ns1/tmp ofs://ozone1/vol1/bucket1/dst

Copy from Ozone to HDFS

When copying files from Ozone to HDFS, similar issues can occur due to differences in checksum types. In this case, you must configure the checksum type for HDFS, as it is the destination system.

Example:

hadoop distcp \
  -Ddfs.checksum.combine.mode=COMPOSITE_CRC \
  -Ddfs.checksum.type=CRC32 \
  ofs://ozone1/vol1/bucket1/src hdfs://ns1/tmp/dst

By specifying the appropriate checksum configuration or skipping the validation, you can ensure that DistCp jobs complete successfully when transferring data between HDFS and Ozone.

Encrypted data

When data resides in an HDFS encryption zone or Ozone encrypted buckets, the file checksum will not match. This is because the underlying block data differs due to the use of a new EDEK (Encryption Data Encryption Key) at the destination. In such cases, specify the -skipcrccheck parameter to avoid job failures.

For more information about using Hadoop DistCp, consult the DistCp Guide.

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