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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1435) Provide an option to store fsimage compressed
Date Thu, 14 Oct 2010 23:29:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12921170#action_12921170

Todd Lipcon commented on HDFS-1435:

Hey Hairong. Another idea which you may want to experiment with at some point is to write
a BufferedInputStream equivalent that does "readahead" or buffer filling in a second thread.
That way the extra CPU caused by compression goes onto another core. Given that the actual
application of the image data to the namespace is single-threaded due to the FSN lock, I bet
compressed reading could actually get faster than uncompressed.

> Provide an option to store fsimage compressed
> ---------------------------------------------
>                 Key: HDFS-1435
>                 URL: https://issues.apache.org/jira/browse/HDFS-1435
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>    Affects Versions: 0.22.0
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>             Fix For: 0.22.0
>         Attachments: checkpoint-limitandcompress.patch, trunkImageCompress.patch, trunkImageCompress1.patch
> Our HDFS has fsimage as big as 20G bytes. It consumes a lot of network bandwidth when
secondary NN uploads a new fsimage to primary NN.
> If we could store fsimage compressed, the problem could be greatly alleviated.
> I plan to provide a new configuration hdfs.image.compressed with a default value of false.
If it is set to be true, fsimage is stored as compressed.
> The fsimage will have a new layout with a new field "compressed" in its header, indicating
if the namespace is stored compressed or not.

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