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From "Colin Patrick McCabe (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 01:39:51 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5722?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13866181#comment-13866181

Colin Patrick McCabe commented on HDFS-5722:

[~tlipcon], [~atm], [~hairong], how do you feel about removing support for on-disk FSImage

It seems to me that we should just add an option for doing HTTP compression, but keep the
old option for on-disk compression.  It concerns me that someone with a small disk might upgrade
to a new version of Hadoop and then be unable to save his (much larger) fsimage on a small
partition once compression support has been removed.  I also think that for really large FSImages,
loading a compressed version could be faster, if the compression were offloaded to a worker
thread like Todd suggested in HDFS-1435.

The FSImage is always read sequentially.  If we implement optional sections, that won't change
this fact.  So I just don't see a reason for messing with this.  But maybe there's something
I have overlooked.


> 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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