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Date: Mon, 3 Aug 2009 13:51:14 -0700 (PDT)
From: "Tsz Wo (Nicholas), SZE (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Subject: [jira] Commented: (HDFS-512) Set block id as the key to Block
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    [ https://issues.apache.org/jira/browse/HDFS-512?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12738577#action_12738577 ] 

Tsz Wo (Nicholas), SZE commented on HDFS-512:
---------------------------------------------

> I was looking at compareTo(), which is used in TreeMap. ...
Even for TreeMap, it is good to implement equals(..) correctly as described by the [TreeMap API|http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html],
bq. .. The behavior of a sorted map is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Map interface.

> By the way, equals() should probably be implemented via compareTo() so that they are always in sync.
Agree.

> Set block id as the key to Block
> --------------------------------
>
>                 Key: HDFS-512
>                 URL: https://issues.apache.org/jira/browse/HDFS-512
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>    Affects Versions: 0.21.0
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>             Fix For: 0.21.0
>
>         Attachments: blockKey.patch
>
>
> Currently the key to Block is block id + generation stamp. I would propose to change it to be only block id. This is based on the following properties of the dfs cluster:
> 1. On each datanode only one replica of block exists. Therefore there is only one generation of a block.
> 2. NameNode has only one entry for a block in its blocks map.
> With this change, search for a block/replica's meta information is easier since most of the time we know a block's id but may not know its generation stamp.

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