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https://issues.apache.org/jira/browse/HDFS-5698?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13882848#comment-13882848
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Daryn Sharp commented on HDFS-5698:
-----------------------------------
I like the premise of this jira but like Kihwal, I too am interested in the
performance/size impacts. My concern is edit logging is already imposing a
non-trivial penalty to throughput. The longer we spend constructing an edit
log entry, the longer the namespace is write locked. The larger the edits, the
fewer edit log entries can be batched synced which both incurs more i/o
penalties and ties up the rpc handler threads longer. It would be nice if you
would gather some statistics.
> 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-5698.000.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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