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From "Andrew Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11486) Client close() should not fail fast if the last block is being decommissioned
Date Fri, 03 Mar 2017 00:54:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15893474#comment-15893474

Andrew Wang commented on HDFS-11486:

Thanks for investigating this Wei-Chiu,

I think this is an issue with the NN's completeFile handling. The {{hasMinStorage}} check
for LIVE nodes excludes D_I_P nodes. This case doesn't come up often since min rep is normally
set to 1, and operators typically decommission a rack at a time so there'll always be one
replica that's not on a D_I_P node.

IMO it's still safe to commit/complete a block even when on a D_I_P node. The DecommissionManager
does a final block scan before transitioning a node from D_I_P to Decommissioned, which will
catch any under-replicated blocks like this. It'd be even better though to add these under-replicated
blocks to the replication queues at close time.

This might also apply to maintenance mode nodes, though I haven't caught up on that work.

> Client close() should not fail fast if the last block is being decommissioned
> -----------------------------------------------------------------------------
>                 Key: HDFS-11486
>                 URL: https://issues.apache.org/jira/browse/HDFS-11486
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 2.6.0
>            Reporter: Wei-Chiu Chuang
>            Assignee: Wei-Chiu Chuang
>         Attachments: HDF-11486.test.patch
> If a DFS client closes a file while the last block is being decommissioned, the close()
may fail if the decommission of the block does not complete in a few seconds.
> When a DataNode is being decommissioned, NameNode marks the DN's state as DECOMMISSION_INPROGRESS_INPROGRESS,
and blocks with replicas on these DataNodes become under-replicated immediately. A close()
call which attempts to complete the last open block will fail if the number of live replicas
is below minimal replicated factor, due to too many replicas residing on the DataNodes.
> The client internally will try to complete the last open block for up to 5 times by default,
which is roughly 12 seconds. After that, close() throws an exception like the following, which
is typically not handled properly.
> {noformat}
> java.io.IOException: Unable to close file because the last blockBP-33575088-
does not have enough number of replicas.
> 	at org.apache.hadoop.hdfs.DFSOutputStream.completeFile(DFSOutputStream.java:864)
> 	at org.apache.hadoop.hdfs.DFSOutputStream.closeImpl(DFSOutputStream.java:827)
> 	at org.apache.hadoop.hdfs.DFSOutputStream.close(DFSOutputStream.java:793)
> 	at org.apache.hadoop.fs.FSDataOutputStream$PositionCache.close(FSDataOutputStream.java:72)
> 	at org.apache.hadoop.fs.FSDataOutputStream.close(FSDataOutputStream.java:101)
> 	at org.apache.hadoop.hdfs.TestDecommission.testCloseWhileDecommission(TestDecommission.java:708)
> 	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
> 	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
> 	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
> 	at java.lang.reflect.Method.invoke(Method.java:498)
> 	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
> 	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
> 	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
> 	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
> 	at org.junit.internal.runners.statements.FailOnTimeout$StatementThread.run(FailOnTimeout.java:74)
> {noformat}
> Once the exception is thrown, the client usually does not attempt to close again, so
the file remains in open state, and the last block remains in under replicated state.
> Subsequently, administrator runs recoverLease tool to salvage the file, but the attempt
failed because the block remains in under replicated state. It is not clear why the block
is never replicated though. However, administrators think it becomes a corrupt file because
the file remains open via fsck -openforwrite and the file modification time is hours ago.
> In summary, I do not think close() should fail because the last block is being decommissioned.
The block has sufficient number replicas, and it's just that some replicas are being decommissioned.
Decomm should be transparent to clients.
> This issue seems to be more prominent on a very large scale cluster, with min replication
factor set to 2.

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