HDFS

Working with files stored in HDFS

You can query files and directories stored in your HDFS cluster. Dremio supports a number of different file formats. To learn more, see the chapter on Files and Directories.

Setup and Best Practices

Colocation

Colocating Dremio nodes with HDFS datanodes can lead to noticeably reduced data transfer times and more performant query execution.

Parquet File Performance

When HDFS data is stored in the Parquet file format, then optimal performance is achieved by storing one Parquet row group per file, with a file size less than or equal to the HDFS block size. Parquet files that overrun the HDFS block size can negatively impact query times by incurring a considerable amount of filesystem overhead.

NOTE: Ensure that your Dremio cluster has access to the appropriate ports for each node of your HDFS source. By default this should be port 8020 for an HDFS NameNode (which should be the one specified when adding the source), and port 50010 for HDFS DataNodes (used internally for data transfer).

Dremio and HDFS

Impersonation and Ownership Chaining

You can enable user-specific file access permissions by turning on impersonation in HDFS sources (check the 'impersonation' box in the source connection dialog). Users who access data stored on an HDFS source with impersonation enabled will have their access mediated by the HDFS privileges associated with their Dremio login name, rather than the ones associated with the Dremio daemon.

For example, let's say a Dremio user named bobsmith has been granted read access to the file /accounts/CustomerA.txt under the same username in HDFS. However, the dremio system user (the user that the dremio daemon runs as) does not have read access to this file. Unless impersonation was enabled when this HDFS source was added to Dremio, bobsmith will be unable to query the file.

Enabling impersonation also permits a kind of behavior called 'ownership chaining.' Under ownership chaining, HDFS data which is subject to restricted access can be shared with any other Dremio users via the creation of a virtual dataset in a public (non-Home) space.

Dremio Configuration

Here are all available source specific options:

Name Description
NameNode Host HDFS NameNode host.
NameNode Port HDFS NameNode port.
Impersonation Enable impersonation.
Root Path Root path for the HDFS source.
Properties A list of additional HDFS connection properties.

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