Wei-Chiu Chuang created HDFS-13103:
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Summary: HDFS Client write acknowledgement timeout should not
depend on read timeout
Key: HDFS-13103
URL: https://issues.apache.org/jira/browse/HDFS-13103
Project: Hadoop HDFS
Issue Type: Bug
Components: datanode, hdfs-client
Affects Versions: 3.0.0-alpha1, 2.8.0
Environment: CDH5.7.0 and above. HBase Region Server.
Reporter: Wei-Chiu Chuang
HDFS-8311 added a timeout for client write acknowledgement for both
# transferring blocks
# writing to a DataNode.
The timeout shares the same configuration as client read timeout
(dfs.client.socket-timeout).
While I agree having a timeout is good, it does not make sense for the write
acknowledgement timeout to depend on read timeout. We saw a case where cluster
admin wants to reduce HBase RegionServer read timeout so as to detect DataNode
crash quickly, but did not realize it affects write acknowledgement timeout.
In the end, the effective DataNode write timeout is shorter than the effective
client write acknowledgement timeout. If the last two DataNodes in the write
pipeline crashes, client would think the first DataNode is faulty (the DN
appears unresponsive because it is still waiting for the ack from downstream
DNs), dropping it and then HBase RS would crash because it is unable to write
to any good DataNode. This scenario is possible during a rack failure.
This problem is even worse for Cloudera Manager-managed cluster. By default,
CM-managed HBase RegionServer sets
{{dfs.client.block.write.replace-datanode-on-failure.enable = true}}. Even one
unresponsive DataNode could crash HBase RegionServer.
I am raising this Jira to discuss two possible solutions
# add a new config for write acknowledgement timeout. Do not depend on read
timeout
# or, update the description of dfs.client.socket-timeout in core-default.xml
so that admin is aware write acknowledgement timeout depends on this
configuration.
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