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https://issues.apache.org/jira/browse/HDFS-16182?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17403006#comment-17403006
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Max Xie commented on HDFS-16182:
---------------------------------
In my cluster, we use BlockPlacementPolicyDefault to choose dn and the number
of SSD DN is much less than DISK DN. It may cause to some block that should
be placed to SSD DNs fallback to place DISK DNs.
> numOfReplicas is given the wrong value in
> BlockPlacementPolicyDefault$chooseTarget can cause DataStreamer to fail with
> Heterogeneous Storage
> -----------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: HDFS-16182
> URL: https://issues.apache.org/jira/browse/HDFS-16182
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namanode
> Affects Versions: 3.4.0
> Reporter: Max Xie
> Assignee: Max Xie
> Priority: Major
> Labels: pull-request-available
> Attachments: HDFS-16182.patch
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> In our hdfs cluster, we use heterogeneous storage to store data in SSD for a
> better performance. Sometimes hdfs client transfer data in pipline, it will
> throw IOException and exit. Exception logs are below:
> ```
> java.io.IOException: Failed to replace a bad datanode on the existing
> pipeline due to no more good datanodes being available to try. (Nodes:
> current=[DatanodeInfoWithStorage[dn01_ip:5004,DS-ef7882e0-427d-4c1e-b9ba-a929fac44fb4,DISK],
>
> DatanodeInfoWithStorage[dn02_ip:5004,DS-3871282a-ad45-4332-866a-f000f9361ecb,DISK],
>
> DatanodeInfoWithStorage[dn03_ip:5004,DS-a388c067-76a4-4014-a16c-ccc49c8da77b,SSD],
>
> DatanodeInfoWithStorage[dn04_ip:5004,DS-b81da262-0dd9-4567-a498-c516fab84fe0,SSD],
>
> DatanodeInfoWithStorage[dn05_ip:5004,DS-34e3af2e-da80-46ac-938c-6a3218a646b9,SSD]],
>
> original=[DatanodeInfoWithStorage[dn01_ip:5004,DS-ef7882e0-427d-4c1e-b9ba-a929fac44fb4,DISK],
>
> DatanodeInfoWithStorage[dn02_ip:5004,DS-3871282a-ad45-4332-866a-f000f9361ecb,DISK]]).
> The current failed datanode replacement policy is DEFAULT, and a client may
> configure this via
> 'dfs.client.block.write.replace-datanode-on-failure.policy' in its
> configuration.
> ```
> After search it, I found when existing pipline need replace new dn to
> transfer data, the client will get one additional dn from namenode and check
> that the number of dn is the original number + 1.
> ```
> ## DataStreamer$findNewDatanode
> if (nodes.length != original.length + 1) {
> throw new IOException(
> "Failed to replace a bad datanode on the existing pipeline "
> + "due to no more good datanodes being available to try. "
> + "(Nodes: current=" + Arrays.asList(nodes)
> + ", original=" + Arrays.asList(original) + "). "
> + "The current failed datanode replacement policy is "
> + dfsClient.dtpReplaceDatanodeOnFailure
> + ", and a client may configure this via '"
> + BlockWrite.ReplaceDatanodeOnFailure.POLICY_KEY
> + "' in its configuration.");
> }
> ```
> The root cause is that Namenode$getAdditionalDatanode returns multi datanodes
> , not one in DataStreamer.addDatanode2ExistingPipeline.
>
> Maybe we can fix it in BlockPlacementPolicyDefault$chooseTarget. I think
> numOfReplicas should not be assigned by requiredStorageTypes.
>
>
>
>
>
>
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