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https://issues.apache.org/jira/browse/HDFS-5722?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13866104#comment-13866104
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Haohui Mai commented on HDFS-5722:
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Indeed the difficulties come from the efficiency prospectives. Currently
skipping N bytes in a compressed stream requires decompressing the data. It can
be problematic because N can be huge. (e.g., when skipping the inode section N
can be as large as a couple GB)
Just to clarify, the code will continue to support old FsImage that has
compression enabled. This jira only proposes to move compression support out of
the new FSImage format.
> 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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