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From "Wei-Chiu Chuang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11160) VolumeScanner incorrectly reports good replicas as corrupt due to race condition
Date Sun, 20 Nov 2016 07:16:58 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15680593#comment-15680593
] 

Wei-Chiu Chuang commented on HDFS-11160:
----------------------------------------

HDFS-11022 is the aftermath of this bug. VolumeScanner detects corruption incorrectly, and
then it reports the older replica gen stamp to NameNode (the replica is updated while VolumeScanner
is scanning).

> VolumeScanner incorrectly reports good replicas as corrupt due to race condition
> --------------------------------------------------------------------------------
>
>                 Key: HDFS-11160
>                 URL: https://issues.apache.org/jira/browse/HDFS-11160
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>         Environment: CDH5.7.4
>            Reporter: Wei-Chiu Chuang
>            Assignee: Wei-Chiu Chuang
>         Attachments: HDFS-11160.reproduce.patch
>
>
> Due to a race condition initially reported in HDFS-6804, VolumeScanner may erroneously
detect good replicas as corrupt. This is serious because in some cases it results in data
loss if all replicas are declared corrupt.
> We are investigating an incidence that caused very high block corruption rate in a relatively
small cluster. Initially, we thought HDFS-11056 is to blame. However, after applying HDFS-11056,
we are still seeing VolumeScanner reporting corrupt replicas.
> It turns out that if a replica is being appended while VolumeScanner is scanning it,
VolumeScanner may use the new checksum to compare against old data, causing checksum mismatch.
> I have a unit test to reproduce the error. Will attach later.
> To fix it, I propose a FinalizedReplica object should also have a lastChecksum field
like ReplicaBeingWritten, and BlockSender should use the in-memory lastChecksum to verify
the partial data in the last chunk on disk. File this jira to discuss a good fix for this
issue.



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