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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