[jira] [Created] (HDFS-14296) Prefer ArrayList over LinkedList in VolumeScanner

["BELUGA BEHR (JIRA)" <ji...@apache.org>]

BELUGA BEHR created HDFS-14296:
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             Summary: Prefer ArrayList over LinkedList in VolumeScanner
                 Key: HDFS-14296
                 URL: https://issues.apache.org/jira/browse/HDFS-14296
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: datanode
    Affects Versions: 3.2.0
            Reporter: BELUGA BEHR
            Assignee: BELUGA BEHR
         Attachments: HDFS-14296.1.patch

{quote}The {{size}}, {{isEmpty}}, {{get}}, {{set}}, {{iterator}}, and {{listIterator}} operations run in constant time. - ArrayList
{quote}

However, for a {{LinkedList}}, the entire list must be traversed to get to the desired index.

Like [Here|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/VolumeScanner.java#L384]

Most of the time, the List is being iterated, which is quicker over a primitive array than walking a LinkedList.

There is one place where an item is removed, potentially from the middle of the list [here|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/VolumeScanner.java#L736] but the speed of removing from the middle of the list isn't bad; it's a system native array shift, and it only happens on the off chance that a block pool is removed from the DataNode.



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