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From "Brahma Reddy Battula (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6937) Another issue in handling checksum errors in write pipeline
Date Sat, 30 Jul 2016 11:07:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15400633#comment-15400633
] 

Brahma Reddy Battula commented on HDFS-6937:
--------------------------------------------

Thanks for [~yzhangal] and [~jojochuang] thanks for deeper look here...

We had come across one issue, where write is failed even 7 DN’s are available due to network
fault at one datanode which is LAST_IN_PIPELINE.
Scenario : (DN3 has N/W Fault and Min repl=2).

Write pipeline:
 DN1->DN2->DN3  => DN3 Gives ERROR_CHECKSUM ack. And so DN2 marked as bad
DN1->DN4-> DN3 => DN3 Gives ERROR_CHECKSUM ack. And so DN4 is marked as bad
….
And so on ( all the times DN3 is LAST_IN_PIPELINE) ... Continued till no more datanodes to
construct the pipeline.

Thinking we can handle like below:

Instead of throwing IOException for ERROR_CHECKSUM ack from downstream, If we can send back
the pipeline ack and client side we can replace both DN2 and DN3 with new nodes as we can’t
decide on which is having network problem.


> Another issue in handling checksum errors in write pipeline
> -----------------------------------------------------------
>
>                 Key: HDFS-6937
>                 URL: https://issues.apache.org/jira/browse/HDFS-6937
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode, hdfs-client
>    Affects Versions: 2.5.0
>            Reporter: Yongjun Zhang
>            Assignee: Wei-Chiu Chuang
>         Attachments: HDFS-6937.001.patch, HDFS-6937.002.patch
>
>
> Given a write pipeline:
> DN1 -> DN2 -> DN3
> DN3 detected cheksum error and terminate, DN2 truncates its replica to the ACKed size.
Then a new pipeline is attempted as
> DN1 -> DN2 -> DN4
> DN4 detects checksum error again. Later when replaced DN4 with DN5 (and so on), it failed
for the same reason. This led to the observation that DN2's data is corrupted. 
> Found that the software currently truncates DN2's replca to the ACKed size after DN3
terminates. But it doesn't check the correctness of the data already written to disk.
> So intuitively, a solution would be, when downstream DN (DN3 here) found checksum error,
propagate this info back to upstream DN (DN2 here), DN2 checks the correctness of the data
already written to disk, and truncate the replica to to MIN(correctDataSize, ACKedSize).
> Found this issue is similar to what was reported by HDFS-3875, and the truncation at
DN2 was actually introduced as part of the HDFS-3875 solution. 
> Filing this jira for the issue reported here. HDFS-3875 was filed by [~tlipcon]
> and found he proposed something similar there.
> {quote}
> if the tail node in the pipeline detects a checksum error, then it returns a special
error code back up the pipeline indicating this (rather than just disconnecting)
> if a non-tail node receives this error code, then it immediately scans its own block
on disk (from the beginning up through the last acked length). If it detects a corruption
on its local copy, then it should assume that it is the faulty one, rather than the downstream
neighbor. If it detects no corruption, then the faulty node is either the downstream mirror
or the network link between the two, and the current behavior is reasonable.
> {quote}
> Thanks.



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