DSSD D5 and Short-Circuit Reads

"Short-circuit" reads bypass the DataNode, allowing a client to read data from the disk directly, as long as the client is co-located with the data. Short-circuit reads provide a substantial performance boost to many applications and help improve HBase random read profile and Impala performance.

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Enabling Short Circuit Reads for HBase and Impala

To enable short-circuit reads for use with DSSD D5 DataNodes, you must have installed the DSSD_SCR parcel during installation. If you have not yet installed this parcel, download, distribute, and activate the parcel. See Parcels. Short-circuit reads are enabled for HBase and Impala by default.

To enable short circuit reads for HBase:
  1. In the Cloudera Manager Admin Console, select Clusters > HBase > Configuration.
  2. Type “short” in the Search box.

    A set of short-circuit read parameters for HBase display.

  3. Select the Enable DSSD Short-Circuit Read property.
  4. You can leave the default values for the Libflood Short-Circuit Read CPU ID , Libflood Short-Circuit Read Command Queues, and Libflood Short-Circuit Read Command Queue Depth parameters or adjust them to fine tune HBase short-circuit reads for best performance.
  5. Click Save Changes to commit the changes.

    The Admin console indicates that there is a stale configuration.

  6. Restart the stale services as indicated. See Stale Configurations.
To enable short-circuit reads for Impala:
  1. In the Cloudera Manager Admin Console, select Clusters > Impala > Configuration.
  2. Type “short” in the Search box.

    A set of short-circuit read parameters for Impala display.

  3. Select the Enable DSSD Short-Circuit Read property.
  4. You can leave the default values for the Libflood Short-Circuit Read CPU ID , Libflood Short-Circuit Read Command Queues, and Libflood Short-Circuit Read Command Queue Depth parameters or adjust them to fine tune Impala short-circuit reads for best performance.
  5. Click Save Changes to commit the changes.

    The Admin console now indicates that there is a stale configuration.

  6. Restart the stale services as indicated. See Stale Configurations.

Disabling Short Circuit Reads for HBase and Impala

Short-circuit reads are enabled for HBase and Impala by default. To disable short-circuit reads for use with DSSD D5 DataNodes:

To disable short circuit reads for HBase:
  1. In the Cloudera Manager Admin Console, select Clusters > HBase > Configuration.
  2. Type “short” in the Search box.

    A set of short-circuit read parameters for HBase display.

  3. Clear the Enable DSSD Short-Circuit Read property.
  4. Click Save Changes to commit the changes.

    The Admin console indicates that there is a stale configuration.

  5. Restart the stale services as indicated. See Stale Configurations.
To disable short-circuit reads for Impala:
  1. In the Cloudera Manager Admin Console, select Clusters > Impala > Configuration.
  2. Type “short” in the Search box.

    A set of short-circuit read parameters for Impala display.

  3. Clear the Enable DSSD Short-Circuit Read property.
  4. Click Save Changes to commit the changes.

    The Admin console now indicates that there is a stale configuration.

  5. Restart the stale services as indicated. See Stale Configurations.