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From "Xiao Chen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11160) VolumeScanner reports write-in-progress replicas as corrupt incorrectly
Date Wed, 14 Dec 2016 18:54:59 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15749130#comment-15749130
] 

Xiao Chen commented on HDFS-11160:
----------------------------------

I was pinged to review this as well. Change makes sense to me, verified test fails without
the BlockSender change.
Test appears to have some duplicate code (wait for scan, and verify info), +1 after that's
cleaned up.

Nice work here Wei-Chiu! Also thanks Yongjun and Kihwal for the reviews.

> VolumeScanner reports write-in-progress replicas as corrupt incorrectly
> -----------------------------------------------------------------------
>
>                 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-11160.001.patch, HDFS-11160.002.patch, HDFS-11160.003.patch,
HDFS-11160.004.patch, HDFS-11160.005.patch, HDFS-11160.006.patch, HDFS-11160.branch-2.patch,
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. This bug is especially prominent when there are
a lot of append requests via HttpFs/WebHDFS.
> 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. A quick and simple fix
is to hold FsDatasetImpl lock and read from disk the checksum.



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