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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1137) Name node is using the write-ahead log improperly
Date Sun, 09 May 2010 06:50:52 GMT

    [ 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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