Replication should be decoupled from heartbeat
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Key: HADOOP-2259
URL: https://issues.apache.org/jira/browse/HADOOP-2259
Project: Hadoop
Issue Type: Bug
Affects Versions: 0.15.0
Environment: Hadoop 80 node cluster
Reporter: Srikanth Kakani
I did a simple experiment of shooting down one node in the cluster and measure
the time taken to replicate the under-replicated blocks.
~30000 blocks were under replicated == ~400 / node should take 200 minutes to
replicate completely given 1 minute heartbeat interval.
My findings: it took around 220 minutes, which is reasonable.
Bug: Replication is coupled with heartbeat. Heartbeat interval is based on how
much a namenode can handle. Repliaction should be based on how much a datanode
can handle.
So given the default heartbeat interval of 20 seconds, we computed datanodes
can handle 2 replications in that interval based on which Namenodes give 2
blocks per heartbeat to replicate.
What we propose is to keep the 20second/2blocks constant and hence a datanode
coming in with a heartbeat of 1 minute interval should be given 6 blocks to
replicate per heartbeat. In this case instead on taking 200 minutes it should
take 200/3 ~1 hour to replicate the entire node.
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