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From "Nathan Roberts (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-7742) favoring decommissioning node for replication can cause a block to stay underreplicated for long periods
Date Fri, 06 Feb 2015 17:23:34 GMT
Nathan Roberts created HDFS-7742:
------------------------------------

             Summary: favoring decommissioning node for replication can cause a block to stay
underreplicated for long periods
                 Key: HDFS-7742
                 URL: https://issues.apache.org/jira/browse/HDFS-7742
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: namenode
    Affects Versions: 2.6.0
            Reporter: Nathan Roberts
            Assignee: Nathan Roberts


When choosing a source node to replicate a block from, a decommissioning node is favored.
The reason for the favoritism is that decommissioning nodes aren't servicing any writes so
in-theory they are less loaded.

However, the same selection algorithm also tries to make sure it doesn't get "stuck" on any
particular node:
{noformat}
      // switch to a different node randomly
      // this to prevent from deterministically selecting the same node even
      // if the node failed to replicate the block on previous iterations
{noformat}
Unfortunately, the decommissioning check is prior to this randomness so the algorithm can
get stuck trying to replicate from a decommissioning node. We've seen this in practice where
a decommissioning datanode was failing to replicate a block for many days, when other viable
replicas of the block were available.

Given that we limit the number of streams we'll assign to a given node (default soft limit
of 2, hard limit of 4), It doesn't seem like favoring a decommissioning node has significant
benefit. i.e. when there is significant replication work to do, we'll quickly hit the stream
limit of the decommissioning nodes and use other nodes in the cluster anyway; when there isn't
significant replication work then in theory we've got plenty of replication bandwidth available
so choosing a decommissioning node isn't much of a win.

I see two choices:
1) Change the algorithm to still favor decommissioning nodes but with some level of randomness
that will avoid always selecting the decommissioning node
2) Remove the favoritism for decommissioning nodes

I prefer #2. It simplifies the algorithm, and given the other throttles we have in place,
I'm not sure there is a significant benefit to selecting decommissioning nodes. 





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