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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5722) Implement compression in the HTTP server of SNN / SBN instead of FSImage
Date Thu, 09 Jan 2014 21:55:51 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5722?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13867125#comment-13867125
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Todd Lipcon commented on HDFS-5722:
-----------------------------------

BTW, if you really see a use case for random-accessing portions of the image, we could put
an uncompressed trailer PB at the end of the file, which contains the section descriptors
with their physical offsets, sizes, and type information. That would allow you to arbitrarily
read a section without having to skip() through the others.

> Implement compression in the HTTP server of SNN / SBN instead of FSImage
> ------------------------------------------------------------------------
>
>                 Key: HDFS-5722
>                 URL: https://issues.apache.org/jira/browse/HDFS-5722
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Haohui Mai
>
> The current FSImage format support compression, there is a field in the header which
specifies the compression codec used to compress the data in the image. The main motivation
was to reduce the number of bytes to be transferred between SNN / SBN / NN.
> The main disadvantage, however, is that it requires the client to access the FSImage
in strictly sequential order. This might not fit well with the new design of FSImage. For
example, serializing the data in protobuf allows the client to quickly skip data that it does
not understand. The compression built-in the format, however, complicates the calculation
of offsets and lengths. Recovering from a corrupted, compressed FSImage is also non-trivial
as off-the-shelf tools like bzip2recover is inapplicable.
> This jira proposes to move the compression from the format of the FSImage to the transport
layer, namely, the HTTP server of SNN / SBN. This design simplifies the format of FSImage,
opens up the opportunity to quickly navigate through the FSImage, and eases the process of
recovery. It also retains the benefits of reducing the number of bytes to be transferred across
the wire since there are compression on the transport layer.



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