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From "Wei-Chiu Chuang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11160) VolumeScanner reports write-in-progress replicas as corrupt incorrectly
Date Tue, 08 Aug 2017 23:58:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11160?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16119207#comment-16119207

Wei-Chiu Chuang commented on HDFS-11160:

Hi [~wheat9] 
an alternative approach is add a retry at client side, so that if client encounters a checksum
error, it retries the read to eliminate the false positive due to the race condition.

I don't mind reverting it from 2.8 branch if it makes large Hadoop operators less concerned
about the release.

> 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
>             Fix For: 2.8.0, 2.7.4, 3.0.0-alpha2
>         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-11160.007.patch, HDFS-11160.008.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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