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https://issues.apache.org/jira/browse/HDFS-16653?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17724075#comment-17724075
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Ayush Saxena commented on HDFS-16653:
-------------------------------------
Committed to trunk.
Thanx [~fujx] for the contribution!!!
> Improve error messages in ShortCircuitCache
> -------------------------------------------
>
> Key: HDFS-16653
> URL: https://issues.apache.org/jira/browse/HDFS-16653
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: dfsadmin
> Affects Versions: 3.1.3
> Environment: Linux version 4.15.0-142-generic
> (buildd@lgw01-amd64-039) (gcc version 5.4.0 20160609 (Ubuntu
> 5.4.0-6ubuntu1~16.04.12))
> Reporter: ECFuzz
> Assignee: ECFuzz
> Priority: Minor
> Labels: pull-request-available
>
>
> {code:java}
> <property>
> <name>dfs.client.mmap.cache.size</name>
> <value>256</value>
> <description>
> When zero-copy reads are used, the DFSClient keeps a cache of recently
> used
> memory mapped regions. This parameter controls the maximum number of
> entries that we will keep in that cache.
> The larger this number is, the more file descriptors we will potentially
> use for memory-mapped files. mmaped files also use virtual address space.
> You may need to increase your ulimit virtual address space limits before
> increasing the client mmap cache size.
>
> Note that you can still do zero-copy reads when this size is set to 0.
> </description>
> </property>
> {code}
> When the configuration item “dfs.client.mmap.cache.size” is set to a negative
> number, it will cause /hadoop/bin hdfs dfsadmin -safemode provides all the
> operation options including enter, leave, get, wait and forceExit are
> invalid, the terminal returns security mode is null and no exceptions are
> thrown.
> In summary, I think we need to improve the check mechanism related to this
> configuration item, *add maxEvictableMmapedSize that is
> "dfs.client.mmap.cache.size" related Precondition check suite error
> message，and give a clear indication when the configuration is abnormal in
> order to solve the problem in time and reduce the impact on the safe mode
> related operations.*
> The details are as follows.
> I think that since the constructor of the ShortCircuitCache class in
> ShortCircuitCache.java in the source code already uses
> Preconditions.checkArgument() to check whether the configuration item value
> is greater than or equal to zero.So when set to a negative number, it will
> lead to the creation of ShortCircuitCache class object in ClientContext.java
> failed.
> But due to Preconditions.checkArgument () in the lack of error information,
> resulting in the terminal using hdfs dfsadmin script appears as follows：
> {code:java}
> hadoop@ljq1:~/hadoop-3.1.3-work/sbin$ hdfs dfsadmin -safemode leave
> safemode: null
> Usage: hdfs dfsadmin [-safemode enter | leave | get | wait | forceExit]
> hadoop@ljq1:~/hadoop-3.1.3-work/sbin$ hdfs dfsadmin -safemode enter
> safemode: null
> Usage: hdfs dfsadmin [-safemode enter | leave | get | wait | forceExit]
> hadoop@ljq1:~/hadoop-3.1.3-work/sbin$ hdfs dfsadmin -safemode get
> safemode: null
> Usage: hdfs dfsadmin [-safemode enter | leave | get | wait | forceExit]
> hadoop@ljq1:~/hadoop-3.1.3-work/sbin$ hdfs dfsadmin -safemode forceExit
> safemode: null
> Usage: hdfs dfsadmin [-safemode enter | leave | get | wait | forceExit]{code}
> And hdfs logs and terminal are not related to the exception thrown.
> Therefore, the cause of the situation can be found directly after adding an
> error message to the original Preconditions.checkArgument(), as follows：
> {code:java}
> hadoop@ljq1:~/hadoop-3.1.3-work/sbin$ hdfs dfsadmin -safemode leave
> safemode: Invalid argument: dfs.client.mmap.cache.size must be greater than
> zero.
> Usage: hdfs dfsadmin [-safemode enter | leave | get | wait | forceExit]{code}
>
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