Hernan Gelaf-Romer created HDFS-17693:
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Summary: De-prioritize reads against EC-enabled decommissioning
datanodes
Key: HDFS-17693
URL: https://issues.apache.org/jira/browse/HDFS-17693
Project: Hadoop HDFS
Issue Type: Improvement
Components: datanode
Reporter: Hernan Gelaf-Romer
At my company, we are running an HDFS cluster with erasure coding enabled.
Frequently, we run into issues when decommissioning datanodes on such clusters.
We've run into two problem scenarios
# We're replacing a non-impaired host. The decommissioning process causes
elevated network I/O on the datanode, which effectively hotspots the datanode
# We're replacing an impaired host. The decommissioning process is slow, and
reads to this host will continue to cause issues until the decommission process
is finished.
Lowering `dfs.namenode.decommission.blocks.per.interval` helps for the first
category of decommisions, but hurts the second, as we want to remove
decommissioning impaired datanodes from the cluster as quickly as possible.
The underlying issue here is that reads are still going to these
decommissioning datanodes. Ideally, it would be great for decommissioning
datanodes to be de-prioritized from the read path, similar to how writes are
de-prioritized.
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