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https://issues.apache.org/jira/browse/HDFS-16322?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17443527#comment-17443527
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Tsz-wo Sze commented on HDFS-16322:
-----------------------------------
> ... However, under concurrency, after the first execution of truncate(...),
> concurrent requests from other clients may append new data and change the
> file length. When truncate(...) is retried after that, it will find the file
> has not been truncated with the same length and truncate it again, which
> causes data loss.
This is not a data loss case. For concurrent truncate and append, NN can
execute them in any order. This case just becomes first append and then
truncate.
> The NameNode implementation of ClientProtocol.truncate(...) can cause data
> loss.
> --------------------------------------------------------------------------------
>
> Key: HDFS-16322
> URL: https://issues.apache.org/jira/browse/HDFS-16322
> Project: Hadoop HDFS
> Issue Type: Bug
> Environment: The runtime environment is Ubuntu 18.04, Java 1.8.0_222
> and Apache Maven 3.6.0.
> The bug can be reproduced by the the testMultipleTruncate() in the
> attachment. First, replace the file TestFileTruncate.java under the directory
> "hadoop-3.3.1-src/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/namenode/"
> with the attachment. Then run "mvn test
> -Dtest=org.apache.hadoop.hdfs.server.namenode.TestFileTruncate#testMultipleTruncate"
> to run the testcase. Finally the "assertFileLength(p, n+newLength)" at 199
> line of TestFileTruncate.java will abort. Because the retry of truncate()
> changes the file size and cause data loss.
> Reporter: nhaorand
> Priority: Major
> Attachments: TestFileTruncate.java
>
>
> The NameNode implementation of ClientProtocol.truncate(...) can cause data
> loss. If dfsclient drops the first response of a truncate RPC call, the retry
> by retry cache will truncate the file again and cause data loss.
> HDFS-7926 avoids repeated execution of truncate(...) by checking if the file
> is already being truncated with the same length. However, under concurrency,
> after the first execution of truncate(...), concurrent requests from other
> clients may append new data and change the file length. When truncate(...) is
> retried after that, it will find the file has not been truncated with the
> same length and truncate it again, which causes data loss.
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