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https://issues.apache.org/jira/browse/HDFS-4660?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13627573#comment-13627573
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PengZhang commented on HDFS-4660:
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#Call hflush to ensure that all DNs have the full length
I think if this process happened, bug will not be triggered.
After client called hflush() without all DNs acked, DN1 may had more bytes than
other DNs.
So if DN2 died and new added DN4 located at 2nd position of the
pipeline(controlled by NM's pipeline sort algorithm), it will recover RBW from
DN1.
After recovery, DN4 may had more bytes than DN3.
And client will continue sending from smallest offset that it received acks.
So this will cause DN4 to "receive a packet, part of which needs to write and
part needs to skip. When amount of data to skip reaches trunk size, receiver
doesn't skip checksum and has it duplicated".
Creating a test case from high-level may be not easy, because we need to
control DNs file position after hflush, and also DN4's location in recovered
pipeline.
> Duplicated checksum on DN in a recovered pipeline
> -------------------------------------------------
>
> Key: HDFS-4660
> URL: https://issues.apache.org/jira/browse/HDFS-4660
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode
> Affects Versions: 3.0.0, 2.0.3-alpha
> Reporter: PengZhang
> Priority: Critical
> Attachments: HDFS-4660.patch
>
>
> pipeline DN1 DN2 DN3
> stop DN2
> pipeline added node DN4 located at 2nd position
> DN1 DN4 DN3
> recover RBW
> DN4 after recover rbw
> 2013-04-01 21:02:31,570 INFO
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl: Recover
> RBW replica
> BP-325305253-10.2.201.14-1364820083462:blk_-9076133543772600337_1004
> 2013-04-01 21:02:31,570 INFO
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl:
> Recovering ReplicaBeingWritten, blk_-9076133543772600337_1004, RBW
> getNumBytes() = 134144
> getBytesOnDisk() = 134144
> getVisibleLength()= 134144
> end at chunk (134144/512=262)
> DN3 after recover rbw
> 2013-04-01 21:02:31,575 INFO
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl: Recover
> RBW replica
> BP-325305253-10.2.201.14-1364820083462:blk_-9076133543772600337_10042013-04-01
> 21:02:31,575 INFO
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl:
> Recovering ReplicaBeingWritten, blk_-9076133543772600337_1004, RBW
> getNumBytes() = 134028
> getBytesOnDisk() = 134028
> getVisibleLength()= 134028
> client send packet after recover pipeline
> offset=133632 len=1008
> DN4 after flush
> 2013-04-01 21:02:31,779 DEBUG
> org.apache.hadoop.hdfs.server.datanode.DataNode: FlushOrsync, file
> offset:134640; meta offset:1063
> // meta end position should be floor(134640/512)*4 + 7 == 1059, but now it is
> 1063.
> DN3 after flush
> 2013-04-01 21:02:31,782 DEBUG
> org.apache.hadoop.hdfs.server.datanode.DataNode: PacketResponder:
> BP-325305253-10.2.201.14-1364820083462:blk_-9076133543772600337_1005,
> type=LAST_IN_PIPELINE, downstreams=0:[]: enqueue Packet(seqno=219,
> lastPacketInBlock=false, offsetInBlock=134640,
> ackEnqueueNanoTime=8817026136871545)
> 2013-04-01 21:02:31,782 DEBUG
> org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl: Changing
> meta file offset of block
> BP-325305253-10.2.201.14-1364820083462:blk_-9076133543772600337_1005 from
> 1055 to 1051
> 2013-04-01 21:02:31,782 DEBUG
> org.apache.hadoop.hdfs.server.datanode.DataNode: FlushOrsync, file
> offset:134640; meta offset:1059
> After checking meta on DN4, I found checksum of chunk 262 is duplicated, but
> data not.
> Later after block was finalized, DN4's scanner detected bad block, and then
> reported it to NM. NM send a command to delete this block, and replicate this
> block from other DN in pipeline to satisfy duplication num.
> I think this is because in BlockReceiver it skips data bytes already written,
> but not skips checksum bytes already written. And function
> adjustCrcFilePosition is only used for last non-completed chunk, but
> not for this situation.
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