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Properties for configuring short-circuit local reads on HDFS

To configure short-circuit local reads, you must add various properties to the hdfs-site.xml file. Short-circuit local reads must be configured on both the DataNode and the client.

Property Name

Property Value

Description

dfs.client.read.shortcircuit

true

Set this to true to enable short-circuit local reads.

dfs.domain.socket.path

/var/lib/hadoop-hdfs/ dn_socket

The path to the domain socket. Short-circuit reads make use of a UNIX domain socket. This is a special path in the file system that allows the client and the DataNodes to communicate. You will need to set a path to this socket. The DataNode needs to be able to create this path. On the other hand, it should not be possible for any user except the hdfs user or root to create this path. For this reason, paths under /var/run or /var/lib are often used.

In the file system that allows the client and the DataNodes to communicate. You will need to set a path to this socket. The DataNode needs to be able to create this path. On the other hand, it should not be possible for any user except the hdfs user or root to create this path. For this reason, paths under /var/run or /var/lib are often used.

dfs.client.domain.socket.data.traffic

false

This property controls whether or not normal data traffic will be passed through the UNIX domain socket. It is recommended that you set the value of this property to false.

Unusual data traffic will be passed through the UNIX domain socket.

dfs.client.use.legacy.blockreader.local

false

Setting this value to false specifies that the new version (based on HDFS-347) of the short-circuit reader is used. Setting this value to true would mean that the legacy short-circuit reader would be used.

dfs.datanode.hdfs-blocks-metadata.enabled

true

Boolean which enables back-end DataNode-side support for the experimental

DistributedFileSystem#getFile

VBlockStorageLocationsAPI.

dfs.client.file-block-storage-locations.timeout

60

Timeout (in seconds) for the parallel RPCs made in

DistributedFileSystem

#getFileBlockStorageLocations().

This property is deprecated but is still supported for backward

compatibility

dfs.client.file-block-storage-locations.timeout.millis

60000

Timeout (in milliseconds) for the parallel RPCs made in

DistributedFileSystem

#getFileBlockStorageLocations().

This property replaces

dfs.client.file-block-storage-locations.timeout,

and offers a finer level of granularity.

dfs.client.read.shortcircuit.skip.checksum

false

If this configuration parameter is set, short-circuit local reads will skip checksums. This is normally not recommended, but it may be useful for special setups. You might consider using this if you are doing your own checksumming outside of HDFS.

dfs.client.read.shortcircuit.streams.cache.size

256

The DFSClient maintains a cache of recently opened file descriptors. This parameter controls the size of that cache. Setting this higher will use more file descriptors, but potentially provide better performance on workloads involving many seeks.

dfs.client.read.shortcircuit.streams.cache.expiry.ms

300000

This controls the minimum amount of time (in milliseconds) file descriptors need to sit in the client cache context before they can be closed for being inactive for too long.

The XML for these entries:

<configuration>

<property>
  <name>dfs.client.read.shortcircuit</name>
  <value>true</value>
</property>
 
<property>
  <name>dfs.domain.socket.path</name>
  <value>/var/lib/hadoop-hdfs/dn_socket</value>
</property>
 
<property>
  <name>dfs.client.domain.socket.data.traffic</name>
  <value>false</value>
</property>
 
<property>
  <name>dfs.client.use.legacy.blockreader.local</name>
  <value>false</value>
</property>
 
<property>
  <name>dfs.datanode.hdfs-blocks-metadata.enabled</name>
  <value>true</value>
</property>
 
<property>
  <name>dfs.client.file-block-storage-locations.timeout.millis</name>
  <value>60000</value>
</property>
 
<property>
  <name>dfs.client.read.shortcircuit.skip.checksum</name>
  <value>false</value>
</property>
 
<property>
  <name>dfs.client.read.shortcircuit.streams.cache.size</name>
  <value>256</value>
</property>
 
<property>
  <name>dfs.client.read.shortcircuit.streams.cache.expiry.ms</name>
  <value>300000</value>
</property>

</configuration>