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https://issues.apache.org/jira/browse/HDFS-15240?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17067539#comment-17067539
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HuangTao commented on HDFS-15240:
---------------------------------
[~weichiu] PTAL, Thanks
> Erasure Coding: dirty buffer causes reconstruction block error
> --------------------------------------------------------------
>
> Key: HDFS-15240
> URL: https://issues.apache.org/jira/browse/HDFS-15240
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode, erasure-coding
> Reporter: HuangTao
> Assignee: HuangTao
> Priority: Major
> Attachments: HDFS-15240.001.patch
>
>
> When read some lzo files we found some blocks were broken.
> I read back all internal blocks(b0-b8) of the block group(RS-6-3-1024k), and
> choose 6(b0-b5) blocks to decode other 3(b6', b7', b8') blocks. And find the
> longest common sequenece(LCS) between b6' and b6(b7'/b7 and b8'/b8).
> After selecting 6 blocks of the block group in combinations one time and
> iterating through all cases, I find one case that the length of LCS is the
> block length - 64KB, 64KB is just the length of ByteBuffer used by
> StripedBlockReader. So the corrupt reconstruction block is made by a dirty
> buffer.
> The following log snippet is my check program output
> {code:java}
> decode from [0, 2, 3, 4, 5, 7] -> [1, 6, 8]
> Check Block(1) first 131072 bytes longest common substring length 4
> Check Block(6) first 131072 bytes longest common substring length 4
> Check Block(8) first 131072 bytes longest common substring length 4
> decode from [0, 2, 3, 4, 5, 6] -> [1, 7, 8]
> Check Block(1) first 131072 bytes longest common substring length 65536
> CHECK AGAIN: Block(1) all 27262976 bytes longest common substring length
> 27197440 # this one
> Check Block(7) first 131072 bytes longest common substring length 4
> Check Block(8) first 131072 bytes longest common substring length 4{code}
> Now I know the dirty buffer causes reconstruction block error, but how does
> the dirty buffer come about?
> After digging into the code and DN log, I found this following DN log is the
> root reason.
> {code:java}
> [INFO] [stripedRead-1017] : Interrupted while waiting for IO on channel
> java.nio.channels.SocketChannel[connected local=/xxxxxxxx:52586
> remote=/xxxxxxxx:50010]. 180000 millis timeout left.
> [WARN] [StripedBlockReconstruction-199] : Failed to reconstruct striped
> block: BP-714356632-xxxxxxxx-1519726836856:blk_-YYYYYYYYYYYYYY_3472979393
> java.lang.NullPointerException
> at
> org.apache.hadoop.hdfs.util.StripedBlockUtil.getNextCompletedStripedRead(StripedBlockUtil.java:314)
> at
> org.apache.hadoop.hdfs.server.datanode.erasurecode.StripedReader.doReadMinimumSources(StripedReader.java:308)
> at
> org.apache.hadoop.hdfs.server.datanode.erasurecode.StripedReader.readMinimumSources(StripedReader.java:269)
> at
> org.apache.hadoop.hdfs.server.datanode.erasurecode.StripedBlockReconstructor.reconstruct(StripedBlockReconstructor.java:94)
> at
> org.apache.hadoop.hdfs.server.datanode.erasurecode.StripedBlockReconstructor.run(StripedBlockReconstructor.java:60)
> at
> java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
> at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
> at
> java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
> at
> java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
> at java.base/java.lang.Thread.run(Thread.java:834) {code}
> Reading from DN may timeout(hold by a future(F)) and output the INFO log, but
> the futures that contains the future(F) is cleared,
> {code:java}
> return new StripingChunkReadResult(futures.remove(future),
> StripingChunkReadResult.CANCELLED); {code}
> futures.remove(future) cause NPE. So the EC reconstruction is failed. In the
> finally phase, the code snippet in *getStripedReader().close()*
> {code:java}
> reconstructor.freeBuffer(reader.getReadBuffer());
> reader.freeReadBuffer();
> reader.closeBlockReader(); {code}
> free buffer firstly, but the StripedBlockReader still holds the buffer and
> write it.
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