[jira] [Commented] (HDFS-3702) Add an option for NOT writing the blocks locally if there is a datanode on the same box as the client

[Tsz Wo Nicholas Sze (JIRA)]

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https://issues.apache.org/jira/browse/HDFS-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15229665#comment-15229665
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Tsz Wo Nicholas Sze commented on HDFS-3702:
-------------------------------------------

BTW, we should check if excludedNodes.contains(writer) is already true; 
otherwise, fallback does not help.  Also, we may set results to null when 
results.size() < numOfReplicas so that the code can be simplified a little bit.
{code}
    boolean avoidLocalNode = addBlockFlags != null
        && addBlockFlags.contains(AddBlockFlag.NO_LOCAL_WRITE)
        && writer != null
        && !excludedNodes.contains(writer);
    List<DatanodeStorageInfo> results = null;
    // Attempt to exclude local node if the client suggests so.
    if (avoidLocalNode) {
      results = new ArrayList<>(chosenStorage);
      Set<Node> excludedNodeCopy = new HashSet<>(excludedNodes);
      excludedNodeCopy.add(writer);
      localNode = chooseTarget(numOfReplicas, writer, excludedNodeCopy,
          blocksize, maxNodesPerRack, results, avoidStaleNodes,
          storagePolicy, EnumSet.noneOf(StorageType.class), results.isEmpty());
      if (results.size() < numOfReplicas) {
        results = null; // not enough nodes; discard results and fall back
      }
    }
    if (results == null) {
      results = new ArrayList<>(chosenStorage);
      localNode = chooseTarget(numOfReplicas, writer, excludedNodes,
          blocksize, maxNodesPerRack, results, avoidStaleNodes,
          storagePolicy, EnumSet.noneOf(StorageType.class), results.isEmpty());
    }
{code}


> Add an option for NOT writing the blocks locally if there is a datanode on 
> the same box as the client
> -----------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-3702
>                 URL: https://issues.apache.org/jira/browse/HDFS-3702
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 2.5.1
>            Reporter: Nicolas Liochon
>            Assignee: Lei (Eddy) Xu
>            Priority: Minor
>              Labels: BB2015-05-TBR
>         Attachments: HDFS-3702.000.patch, HDFS-3702.001.patch, 
> HDFS-3702.002.patch, HDFS-3702.003.patch, HDFS-3702.004.patch, 
> HDFS-3702.005.patch, HDFS-3702.006.patch, HDFS-3702.007.patch, 
> HDFS-3702.008.patch, HDFS-3702.009.patch, HDFS-3702.010.patch, 
> HDFS-3702.011.patch, HDFS-3702_Design.pdf
>
>
> This is useful for Write-Ahead-Logs: these files are writen for recovery 
> only, and are not read when there are no failures.
> Taking HBase as an example, these files will be read only if the process that 
> wrote them (the 'HBase regionserver') dies. This will likely come from a 
> hardware failure, hence the corresponding datanode will be dead as well. So 
> we're writing 3 replicas, but in reality only 2 of them are really useful.



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