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From "Hudson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8797) WebHdfsFileSystem creates too many connections for pread
Date Thu, 23 Jul 2015 11:57:07 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14638696#comment-14638696
] 

Hudson commented on HDFS-8797:
------------------------------

SUCCESS: Integrated in Hadoop-Yarn-trunk-Java8 #265 (See [https://builds.apache.org/job/Hadoop-Yarn-trunk-Java8/265/])
HDFS-8797. WebHdfsFileSystem creates too many connections for pread. Contributed by Jing Zhao.
(jing9: rev e91ccfad07ec5b5674a84009772dd31a82b4e4de)
* hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/web/TestWebHDFS.java
* hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/web/ByteRangeInputStream.java
* hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/web/TestByteRangeInputStream.java
* hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt


> WebHdfsFileSystem creates too many connections for pread
> --------------------------------------------------------
>
>                 Key: HDFS-8797
>                 URL: https://issues.apache.org/jira/browse/HDFS-8797
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: webhdfs
>            Reporter: Jing Zhao
>            Assignee: Jing Zhao
>             Fix For: 2.8.0
>
>         Attachments: HDFS-8797.000.patch, HDFS-8797.001.patch, HDFS-8797.002.patch, HDFS-8797.003.patch
>
>
> While running a test we found that WebHdfsFileSystem can create several thousand connections
when doing a position read of a 200MB file. For each connection the client will connect to
the DataNode again and the DataNode will create a new DFSClient instance to handle the read
request. This also leads to several thousand {{getBlockLocations}} call to the NameNode.
> The cause of the issue is that in {{FSInputStream#read(long, byte[], int, int)}}, each
time the inputstream reads some time, it seeks back to the old position and resets its state
to SEEK. Thus the next read will regenerate the connection.
> {code}
>   public int read(long position, byte[] buffer, int offset, int length)
>     throws IOException {
>     synchronized (this) {
>       long oldPos = getPos();
>       int nread = -1;
>       try {
>         seek(position);
>         nread = read(buffer, offset, length);
>       } finally {
>         seek(oldPos);
>       }
>       return nread;
>     }
>   }
> {code}



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