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From "Yongjun Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9600) do not check replication if the block is under construction
Date Sat, 06 Feb 2016 00:56:39 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9600?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15135372#comment-15135372
] 

Yongjun Zhang commented on HDFS-9600:
-------------------------------------

HI [~yangzhe1991], [~szetszwo] and [~vinayrpet],

Thanks for your earlier work here. I noticed that in branch-2.6 

{code}
  /**
   * Return true if there are any blocks on this node that have not
   * yet reached their replication factor. Otherwise returns false.
   */
  boolean isReplicationInProgress(DatanodeDescriptor srcNode) {
    boolean status = false;
    boolean firstReplicationLog = true;
    int underReplicatedBlocks = 0;
    int decommissionOnlyReplicas = 0;
    int underReplicatedInOpenFiles = 0;
    final Iterator<? extends Block> it = srcNode.getBlockIterator();
    while(it.hasNext()) {
      final Block block = it.next();
      BlockCollection bc = blocksMap.getBlockCollection(block);

      if (bc != null) {
        NumberReplicas num = countNodes(block);
        int curReplicas = num.liveReplicas();
        int curExpectedReplicas = getReplication(block);

        if (curReplicas < curExpectedReplicas
            || !isPlacementPolicySatisfied(block)) {
{code}

And clause {{ blockInfo.isComplete()}} is added to the following method but not in the above
method, Would any of you please explain why {{blockInfo.isComplete()}}  is not needed in the
above code? Thanks a lot.

{code}
  boolean isNeededReplication(Block b, int expected, int current) {
    BlockInfo blockInfo;
    if (b instanceof BlockInfo) {
      blockInfo = (BlockInfo) b;
    } else {
      blockInfo = getStoredBlock(b);
    }
    return blockInfo.isComplete()
        && (current < expected || !isPlacementPolicySatisfied(b));
  }
{code}



> do not check replication if the block is under construction
> -----------------------------------------------------------
>
>                 Key: HDFS-9600
>                 URL: https://issues.apache.org/jira/browse/HDFS-9600
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: Phil Yang
>            Assignee: Phil Yang
>            Priority: Critical
>             Fix For: 2.8.0, 2.7.3, 2.6.4
>
>         Attachments: HDFS-9600-branch-2.6.patch, HDFS-9600-branch-2.7.patch, HDFS-9600-branch-2.patch,
HDFS-9600-v1.patch, HDFS-9600-v2.patch, HDFS-9600-v3.patch, HDFS-9600-v4.patch
>
>
> When appending a file, we will update pipeline to bump a new GS and the old GS will be
considered as out of date. When changing GS, in BlockInfo.setGenerationStampAndVerifyReplicas
we will remove replicas having old GS which means we will remove all replicas because no DN
has new GS until the block with new GS is added to blockMaps again by DatanodeProtocol.blockReceivedAndDeleted.
> If we check replication of this block before it is added back, it will be regarded as
missing. The probability is low but if there are decommissioning nodes the DecommissionManager.Monitor
will scan all blocks belongs to decommissioning nodes with a very fast speed so the probability
of finding missing block is very high but actually they are not missing. 
> Furthermore, after closing the appended file, in FSNamesystem.finalizeINodeFileUnderConstruction,
it will checkReplication. If some of nodes are decommissioning, this block with new GS will
be added to UnderReplicatedBlocks map so there are two blocks with same ID in this map, one
is in QUEUE_WITH_CORRUPT_BLOCKS and the other is in QUEUE_HIGHEST_PRIORITY or QUEUE_UNDER_REPLICATED.
And there will be many missing blocks warning in NameNode website but there is no corrupt
files...
> Therefore, I think the solution is we should not check replication if the block is under
construction. We only check complete blocks.



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