Improve Namenode robustness by prioritizing datanode heartbeats over client
requests
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Key: HDFS-599
URL: https://issues.apache.org/jira/browse/HDFS-599
Project: Hadoop HDFS
Issue Type: Improvement
Components: name-node
Reporter: dhruba borthakur
Assignee: dhruba borthakur
The namenode processes RPC requests from clients that are reading/writing to
files as well as heartbeats/block reports from datanodes.
Sometime, because of various reasons (Java GC runs, inconsistent performance of
NFS filer that stores HDFS transacttion logs, etc), the namenode encounters
transient slowness. For example, if the device that stores the HDFS transaction
logs becomes sluggish, the Namenode's ability to process RPCs slows down to a
certain extent. During this time, the RPCs from clients as well as the RPCs
from datanodes suffer in similar fashion. If the underlying problem becomes
worse, the NN's ability to process a heartbeat from a DN is severly impacted,
thus causing the NN to declare that the DN is dead. Then the NN starts
replicating blocks that used to reside on the now-declared-dead datanode. This
adds extra load to the NN. Then the now-declared-datanode finally
re-establishes contact with the NN, and sends a block report. The block report
processing on the NN is another heavyweight activity, thus casing more load to
the already overloaded namenode.
My proposal is tha the NN should try its best to continue processing RPCs from
datanodes and give lesser priority to serving client requests. The Datanode
RPCs are integral to the consistency and performance of the Hadoop file system,
and it is better to protect it at all costs. This will ensure that NN recovers
from the hiccup much faster than what it does now.
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