Chen Liang created HDFS-11419:
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Summary: 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
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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