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https://issues.apache.org/jira/browse/HDFS-10490?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15317587#comment-15317587
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He Tianyi commented on HDFS-10490:
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Thanks for pointing that out, [~kihwal].
I think the case is a little different here. The catch is whether DataNode
flushed metafile header to disk or not.
If not (as in this case, output stream for metafile is not properly closed yet,
for there is no way for DataNode to actively detect a network problem),
{{BlockSender}} will see a 0-length metafile (not even containing header), then
use (NULL, 512) for checksuming, client will still fail, given that
configurated bytesPerChecksum != 512.
> Client may never recovery replica after a timeout during sending packet
> -----------------------------------------------------------------------
>
> Key: HDFS-10490
> URL: https://issues.apache.org/jira/browse/HDFS-10490
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode
> Affects Versions: 2.6.0
> Reporter: He Tianyi
> Attachments: HDFS-10490.patch
>
>
> For newly created replica, a meta file is created in constructor of
> {{BlockReceiver}} (for {{WRITE_BLOCK}} op). Its header will be written lazily
> (buffered in memory first by {{BufferedOutputStream}}).
> If following packets fail to deliver (e.g. in extreme network condition),
> the header may never get flush until closed.
> However, {{BlockReceiver}} will not call close until block receiving is
> finished or exception(s) encountered. Also in extreme network condition, both
> RST & FIN may not deliver in time.
> In this case, if client tries to initiates a {{transferBlock}} to a new
> datanode (in {{addDatanode2ExistingPipeline}}), existing datanode will see an
> empty meta if its {{BlockReceiver}} did not close in time.
> Then, after HDFS-3429, a default {{DataChecksum}} (NULL, 512) will be used
> during transfer. So when client then tries to recover pipeline after
> completely transferred, it may encounter the following exception:
> {noformat}
> java.io.IOException: Client requested checksum DataChecksum(type=CRC32C,
> chunkSize=4096) when appending to an existing block with different chunk
> size: DataChecksum(type=NULL, chunkSize=512)
> at
> org.apache.hadoop.hdfs.server.datanode.ReplicaInPipeline.createStreams(ReplicaInPipeline.java:230)
> at
> org.apache.hadoop.hdfs.server.datanode.BlockReceiver.<init>(BlockReceiver.java:226)
> at
> org.apache.hadoop.hdfs.server.datanode.DataXceiver.writeBlock(DataXceiver.java:798)
> at
> org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opWriteBlock(Receiver.java:166)
> at
> org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:76)
> at
> org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:243)
> at java.lang.Thread.run(Thread.java:745)
> {noformat}
> This will repeat, until exhausted by datanode replacement policy.
> Also to note that, with bad luck (like I), 20k clients are all doing this.
> It's to some extend a DDoS attack to NameNode (because of
> getAdditionalDataNode calls).
> I suggest we flush immediately after header is written, preventing anybody
> from seeing empty meta file for avoiding the issue.
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