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https://issues.apache.org/jira/browse/HDFS-7435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14230191#comment-14230191
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Suresh Srinivas commented on HDFS-7435:
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bq. The in-memory representation of the blocks list is merely an implementation
detail that should not influence the PB encoding. Ie. Leave blocksBuffer as a
non-repeating field, and let the NN chunk the block list however it sees fit.
I think the chunked buffers should be sent all the way from DataNode to the
NameNode. NameNode can store it in a different way that fits it needs (though
if it is chunked, I do not see NameNode storing it differently). This is
important because the problems of large contiguous array is also a problem for
DataNode, since there are deployments that use 60 disks in a single node with
more than 10 million blocks in a single DataNode.
> PB encoding of block reports is very inefficient
> ------------------------------------------------
>
> Key: HDFS-7435
> URL: https://issues.apache.org/jira/browse/HDFS-7435
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode, namenode
> Affects Versions: 2.0.0-alpha, 3.0.0
> Reporter: Daryn Sharp
> Assignee: Daryn Sharp
> Priority: Critical
> Attachments: HDFS-7435.000.patch, HDFS-7435.001.patch, HDFS-7435.patch
>
>
> Block reports are encoded as a PB repeating long. Repeating fields use an
> {{ArrayList}} with default capacity of 10. A block report containing tens or
> hundreds of thousand of longs (3 for each replica) is extremely expensive
> since the {{ArrayList}} must realloc many times. Also, decoding repeating
> fields will box the primitive longs which must then be unboxed.
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