Hadoop 2.4 introduced a new feature called hedged reads. If a read from a block is slow, the HDFS client starts up another parallel, 'hedged' read against a different block replica. The result of whichever read returns first is used, and the outstanding read is cancelled. This feature helps in situations where a read occasionally takes a long time rather than when there is a systemic problem. Hedged reads can be enabled for HBase when the HFiles are stored in HDFS. This feature is disabled by default.
Enabling Hedged Reads for HBase
Minimum Required Role: Configurator (also provided by Cluster Administrator, Full Administrator)
- Go to the HBase service.
- Click the Configuration tab.
- Select .
- Select .
- Configure the HDFS Hedged Read Threadpool Size and HDFS Hedged Read Delay Threshold properties. The descriptions for each of these properties on the configuration pages provide more information.
- Enter a Reason for change, and then click Save Changes to commit the changes.
Monitoring the Performance of Hedged Reads
- hedgedReadOps - the number of hedged reads that have occurred
- hedgeReadOpsWin - the number of times the hedged read returned faster than the original read