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https://issues.apache.org/jira/browse/HDFS-1368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12915915#action_12915915
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Suresh Srinivas commented on HDFS-1368:
---------------------------------------

The patch in this jira changed DatanodeDescriptor#numBlock() as:
{noformat}
-    return blockList == null ? 0 : blockList.listCount(this);
+    return numBlocks;
{noformat}

so I did mean BlockInfo#listCount(DatanodeDescriptor).

> Add a block counter to DatanodeDescriptor
> -----------------------------------------
>
>                 Key: HDFS-1368
>                 URL: https://issues.apache.org/jira/browse/HDFS-1368
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>             Fix For: 0.22.0
>
>         Attachments: blockCounter.patch
>
>
> Many of time, we need to count the total number of blocks in a datanode by 
> calling DatanodeDescriptor#numBlocks(). It has to traverse the block list to 
> get the counter, which may not be trivial if there are millions of blocks. 
> Adding a counter for each datanode does not cost much space but help save CPU 
> processing time.

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