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Date: Sun, 9 May 2010 02:50:52 -0400 (EDT)
From: "dhruba borthakur (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Subject: [jira] Commented: (HDFS-1137) Name node is using the write-ahead
 log improperly
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    [ https://issues.apache.org/jira/browse/HDFS-1137?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12865563#action_12865563 ] 

dhruba borthakur commented on HDFS-1137:
----------------------------------------

@Ben, let's walk through with an example. Suppose a request comes in to create a file foo.txt. Now, we need to insert it into the namespace and we need to ensure that new calls to create the same file should fail. When the first request arrives, we have to insert  it into some kind of data structure, so that the next suceeding call detects that a file creation operation for the same file is in progress, isn't it?

> Name node is using the write-ahead log improperly
> -------------------------------------------------
>
>                 Key: HDFS-1137
>                 URL: https://issues.apache.org/jira/browse/HDFS-1137
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: name-node
>            Reporter: Benjamin Reed
>
> The Name node is doing the write-ahead log (WAL) (aka edit log) improperly. Usually when using WAL, changes are written to the log before they are applied to the state. Currently the Namenode does the WAL after applying the change. This means that read may see changes before they are durable. A client may read information and the server fail before the information is written to the WAL, which results in the client reading state that disappears. To fix the Namenode should write changes before (aka ahead of) applying the change.

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