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From "Wei-Chiu Chuang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11472) Fix inconsistent replica size after a data pipeline failure
Date Mon, 05 Jun 2017 19:10:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11472?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16037419#comment-16037419

Wei-Chiu Chuang commented on HDFS-11472:

Hi [~xkrogen]. Thanks for the comment.
I think there's no harm adding the extra warning, as it might still be possible a similar
error creeps into it even after this fix.

I am not so sure about resetting the replica's in-memory last chunk checksum. After the recovery
initiated by {{initReplicaRecoveryImpl}}, the block may be read, and if the LCC does not match
the data in the last chunk, the reader would erroneously believe the block is corrupt, which
defeats the purpose of the fix.

The reason that {{recoverRbwImpl}} resets the replica's in-memory LCC to null, is that after
the recovery, the block is immediately being written, so the LCC won't match the chunk data
anyway (after the block is finalized, LCC is updated), and there is little benefit in making
the LCC correct.

> Fix inconsistent replica size after a data pipeline failure
> -----------------------------------------------------------
>                 Key: HDFS-11472
>                 URL: https://issues.apache.org/jira/browse/HDFS-11472
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>            Reporter: Wei-Chiu Chuang
>            Assignee: Wei-Chiu Chuang
>            Priority: Critical
>              Labels: release-blocker
>         Attachments: HDFS-11472.001.patch, HDFS-11472.002.patch, HDFS-11472.003.patch,
> We observed a case where a replica's on disk length is less than acknowledged length,
breaking the assumption in recovery code.
> {noformat}
> 2017-01-08 01:41:03,532 WARN org.apache.hadoop.hdfs.server.protocol.InterDatanodeProtocol:
Failed to obtain replica info for block (=BP-947993742-
from datanode (=DatanodeInfoWithStorage[,null,null])
> java.io.IOException: THIS IS NOT SUPPOSED TO HAPPEN: getBytesOnDisk() < getVisibleLength(),
rip=ReplicaBeingWritten, blk_2526438952_1101394519586, RBW
>   getNumBytes()     = 27530
>   getBytesOnDisk()  = 27006
>   getVisibleLength()= 27268
>   getVolume()       = /data/6/hdfs/datanode/current
>   getBlockFile()    = /data/6/hdfs/datanode/current/BP-947993742-
>   bytesAcked=27268
>   bytesOnDisk=27006
>         at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.initReplicaRecovery(FsDatasetImpl.java:2284)
>         at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.initReplicaRecovery(FsDatasetImpl.java:2260)
>         at org.apache.hadoop.hdfs.server.datanode.DataNode.initReplicaRecovery(DataNode.java:2566)
>         at org.apache.hadoop.hdfs.server.datanode.DataNode.callInitReplicaRecovery(DataNode.java:2577)
>         at org.apache.hadoop.hdfs.server.datanode.DataNode.recoverBlock(DataNode.java:2645)
>         at org.apache.hadoop.hdfs.server.datanode.DataNode.access$400(DataNode.java:245)
>         at org.apache.hadoop.hdfs.server.datanode.DataNode$5.run(DataNode.java:2551)
>         at java.lang.Thread.run(Thread.java:745)
> {noformat}
> It turns out that if an exception is thrown within {{BlockReceiver#receivePacket}}, the
in-memory replica on disk length may not be updated, but the data is written to disk anyway.
> For example, here's one exception we observed
> {noformat}
> 2017-01-08 01:40:59,512 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Exception
for BP-947993742-
> java.nio.channels.ClosedByInterruptException
>         at java.nio.channels.spi.AbstractInterruptibleChannel.end(AbstractInterruptibleChannel.java:202)
>         at sun.nio.ch.FileChannelImpl.position(FileChannelImpl.java:269)
>         at org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl.adjustCrcChannelPosition(FsDatasetImpl.java:1484)
>         at org.apache.hadoop.hdfs.server.datanode.BlockReceiver.adjustCrcFilePosition(BlockReceiver.java:994)
>         at org.apache.hadoop.hdfs.server.datanode.BlockReceiver.receivePacket(BlockReceiver.java:670)
>         at org.apache.hadoop.hdfs.server.datanode.BlockReceiver.receiveBlock(BlockReceiver.java:857)
>         at org.apache.hadoop.hdfs.server.datanode.DataXceiver.writeBlock(DataXceiver.java:797)
>         at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.opWriteBlock(Receiver.java:169)
>         at org.apache.hadoop.hdfs.protocol.datatransfer.Receiver.processOp(Receiver.java:106)
>         at org.apache.hadoop.hdfs.server.datanode.DataXceiver.run(DataXceiver.java:244)
>         at java.lang.Thread.run(Thread.java:745)
> {noformat}
> There are potentially other places and causes where an exception is thrown within {{BlockReceiver#receivePacket}},
so it may not make much sense to alleviate it for this particular exception. Instead, we should
improve replica recovery code to handle the case where ondisk size is less than acknowledged
size, and update in-memory checksum accordingly.

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