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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 Tue, 28 Feb 2017 18:50:45 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11472?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15888655#comment-15888655
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Wei-Chiu Chuang commented on HDFS-11472:
----------------------------------------

Inviting folks to comment. It may not be a good idea to be too clever about the error handling,
but we had a data loss incidence due to this issue, so I want to see if we can alleviate this
problem using this approach.

> 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
>         Attachments: HDFS-11472.testcase.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-947993742-10.204.0.136-1362248978912:blk_2526438952_1101394519586)
from datanode (=DatanodeInfoWithStorage[10.204.138.17:1004,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-947993742-10.204.0.136-1362248978912/current/rbw/blk_2526438952
>   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-947993742-10.204.0.136-1362248978912:blk_2526438952_1101394499067
> 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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