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https://issues.apache.org/jira/browse/HDFS-5698?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13891348#comment-13891348
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Haohui Mai commented on HDFS-5698:
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Thanks very much for the detailed comments from [~tlipcon].
I've filed HDFS-5884, HDFS-5885 and HDFS-5887 to address the comments.
Thanks very much for the suggestions of the performance improvement. I'll dig
into it.
My plan is to commit HDFS-5884 and HDFS-5885 before the merge, and to continue
to improve the code in trunk. Does it make sense for you?
bq. would existing ImageVisitor implementation classes continue to work
against the PB-ified image?
The existing ImageVisitor implementation won't work with the PB FSImage.
> Use protobuf to serialize / deserialize FSImage
> -----------------------------------------------
>
> Key: HDFS-5698
> URL: https://issues.apache.org/jira/browse/HDFS-5698
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Haohui Mai
> Assignee: Haohui Mai
> Attachments: HDFS-5698-design.pdf, HDFS-5698.000.patch,
> HDFS-5698.001.patch, HDFS-5698.002.patch, HDFS-5698.003.patch
>
>
> Currently, the code serializes FSImage using in-house serialization
> mechanisms. There are a couple disadvantages of the current approach:
> # Mixing the responsibility of reconstruction and serialization /
> deserialization. The current code paths of serialization / deserialization
> have spent a lot of effort on maintaining compatibility. What is worse is
> that they are mixed with the complex logic of reconstructing the namespace,
> making the code difficult to follow.
> # Poor documentation of the current FSImage format. The format of the FSImage
> is practically defined by the implementation. An bug in implementation means
> a bug in the specification. Furthermore, it also makes writing third-party
> tools quite difficult.
> # Changing schemas is non-trivial. Adding a field in FSImage requires bumping
> the layout version every time. Bumping out layout version requires (1) the
> users to explicitly upgrade the clusters, and (2) putting new code to
> maintain backward compatibility.
> This jira proposes to use protobuf to serialize the FSImage. Protobuf has
> been used to serialize / deserialize the RPC message in Hadoop.
> Protobuf addresses all the above problems. It clearly separates the
> responsibility of serialization and reconstructing the namespace. The
> protobuf files document the current format of the FSImage. The developers now
> can add optional fields with ease, since the old code can always read the new
> FSImage.
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