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https://issues.apache.org/jira/browse/HADOOP-2330?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12624427#action_12624427
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dhruba borthakur commented on HADOOP-2330:
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We should not assume that when a file is preallocated by seeking/writing to a
size beyond the current file size that the intermediate data will be filled
with zeros. This portion of the file is actually unspecified.
http://java.sun.com/j2se/1.4.2/docs/api/java/nio/channels/FileChannel.html#position(long)
So, the trick you suggest of setting INVALID_OP to zero might not work
correctly.
> Preallocate transaction log to improve namenode transaction logging
> performance
> -------------------------------------------------------------------------------
>
> Key: HADOOP-2330
> URL: https://issues.apache.org/jira/browse/HADOOP-2330
> Project: Hadoop Core
> Issue Type: Improvement
> Components: dfs
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
> Attachments: preallocateTransactionLog.patch,
> preallocateTransactionLog.patch, preallocateTransactionLog2.patch,
> preallocateTransactionLog3.patch
>
>
> In the current implementation, the transaction log is opened in "append" mode
> and every new transaction is written to the end of the log. This means that
> new blocks get allocated to the edits file frequently.
> It is worth measuring the performance improvement when big chunks of the
> transaction log are allocated up front. Adding new transactions do not cause
> frequent block allocations for the edits log.
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