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From "Yongjun Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6937) Another issue in handling checksum errors in write pipeline
Date Thu, 26 May 2016 08:15:15 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15301733#comment-15301733

Yongjun Zhang commented on HDFS-6937:

Attached a draft patch that reports

2016-05-25 11:35:20,639 [DataStreamer for file /tmp/testClientReportBadBlock/CorruptTwoOutOfThreeReplicas1
block BP-198184347-] WARN  hdfs.DataStreamer (DataStreamer.java:handleBadDatanode(1400))
- Error Recovery for BP-198184347- in pipeline
[DatanodeInfoWithStorage[,DS-d6b01513-ac11-4fdf-99a1-fbb111d0f0c5,DISK], DatanodeInfoWithStorage[,DS-67174cd1-1f9c-46bc-9dea-8ece7190308d,DISK],
DatanodeInfoWithStorage[,DS-8fe30be4-1244-4059-a567-bc156d49d01a,DISK]]: datanode
0(DatanodeInfoWithStorage[,DS-d6b01513-ac11-4fdf-99a1-fbb111d0f0c5,DISK]) is
where is DN3 the reported example.

With the fix, we can see

2016-05-25 14:15:29,831 [DataXceiver for client DFSClient_NONMAPREDUCE_1623233781_1 at /
[Receiving block BP-1085730607-]] WARN  datanode.DataNode
(DataXceiver.java:determineFirstBadLink(494)) - Datanode 2 got response for connect ack  from
downstream datanode with firstbadlink as, however, the replica on the current
Datanode host4:38574 is found to be corrupted, set the firstBadLink to this DataNode.
where host4:38574 is DN2 in the example. Thus it's reported bad in Error Recovery message:

2016-05-25 14:15:29,833 [DataStreamer for file /tmp/testClientReportBadBlock/CorruptTwoOutOfThreeReplicas1
block BP-1085730607-] WARN  hdfs.DataStreamer (DataStreamer.java:handleBadDatanode(1400))
- Error Recovery for BP-1085730607- in pipeline
[DatanodeInfoWithStorage[,DS-a743f66a-3379-4a1e-82df-5f6f26815df8,DISK], DatanodeInfoWithStorage[,DS-32e47236-c29f-435b-995b-f6f4f2a86acc,DISK],
DatanodeInfoWithStorage[,DS-906d823d-0de5-40f1-9409-4c8c6d4edd08,DISK]]: datanode
0(DatanodeInfoWithStorage[,DS-a743f66a-3379-4a1e-82df-5f6f26815df8,DISK]) is

I was able to see the fix constantly succeed in one debug version, and fail when removing
the fix. However, before I post the patch now, I observed some intermittency in the unit test
env, which is yet to understood. But I'd like to post the patch to let it rolling.

Hi [~cmccabe],

Would you please help taking a look? 

Thanks a lot.

> 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: Yongjun Zhang
>         Attachments: HDFS-6937.001.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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