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https://issues.apache.org/jira/browse/HDFS-11419?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15870528#comment-15870528
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Chen Liang commented on HDFS-11419:
-----------------------------------
[~linyiqun] thanks for the comments!
I think you are right about the excluded nodes part. Now it appears to me that
if the randomly chosen node could not satisfy storage type requirement, there
is nothing done to that node...(no {{else}} branch for {{storage == null}}
case) which means it could be randomly picked again...In which case it works in
a similar way like excluded node set: when a node is chosen, it could be
satisfying storage type policy, but it is already picked (in excluded nodes
set), OR it simply does not satisfy storage type. Either way we ignore it and
try again. Only a node not in either of these two situations can pass.
Will change the description to make this correction. Thanks for the catch!
> BlockPlacementPolicyDefault is choosing datanode in an inefficient way
> ----------------------------------------------------------------------
>
> Key: HDFS-11419
> URL: https://issues.apache.org/jira/browse/HDFS-11419
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Reporter: Chen Liang
> Assignee: Chen Liang
>
> Currently in {{BlockPlacementPolicyDefault}}, {{chooseTarget}} will end up
> calling into {{chooseRandom}}, which will first find a random datanode by
> calling
> {code}DatanodeDescriptor chosenNode = chooseDataNode(scope,
> excludedNodes);{code}, then it checks whether that returned datanode
> satisfies storage type requirement
> {code}storage = chooseStorage4Block(
> chosenNode, blocksize, results, entry.getKey());{code}
> If yes, {{numOfReplicas--;}}, otherwise, the node is added to excluded nodes,
> and runs the loop again until {{numOfReplicas}} is down to 0.
> A problem here is that, storage type is not being considered only until after
> a random node is already returned. We've seen a case where a cluster has a
> large number of datanodes, while only a few satisfy the storage type
> condition. So, for the most part, this code blindly picks random datanodes
> that do not satisfy the storage type, and adds the node to excluded and tries
> again and again.
> To make matters worse, the way {{NetworkTopology#chooseRandom}} works is
> that, given a set of excluded nodes, it first finds a random datanodes, then
> if it is in excluded nodes set, try find another random nodes. So the more
> excluded nodes there are, the more likely a random node will be in the
> excluded set, in which case we basically wasted one iteration.
> Therefore, this JIRA proposes to augment/modify the relevant classes in a way
> that datanodes can be found more efficiently. There are currently two
> different high level solutions we are considering:
> 1. add some field to Node base types to describe the storage type info, and
> when searching for a node, we take such field(s) in to account, and do not
> return node that does not meet the storage type requirement.
> 2. change {{NetworkTopology}} class to be aware of storage types: for one
> storage type, there is one tree subset that connects all the nodes with that
> type. And one search happens on only one such subset. So unexpected storage
> types are simply in the search space.
> Thanks [~szetszwo] for the offline discussion, and any comments are more than
> welcome.
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