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https://issues.apache.org/jira/browse/HDFS-9079?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15281208#comment-15281208
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Uma Maheswara Rao G commented on HDFS-9079:
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[~zhz], could you please take a look at find bug comments?
> Erasure coding: preallocate multiple generation stamps and serialize updates
> from data streamers
> ------------------------------------------------------------------------------------------------
>
> Key: HDFS-9079
> URL: https://issues.apache.org/jira/browse/HDFS-9079
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: erasure-coding
> Affects Versions: HDFS-7285
> Reporter: Zhe Zhang
> Assignee: Zhe Zhang
> Attachments: HDFS-9079-HDFS-7285.00.patch, HDFS-9079.01.patch,
> HDFS-9079.02.patch, HDFS-9079.03.patch, HDFS-9079.04.patch,
> HDFS-9079.05.patch, HDFS-9079.06.patch, HDFS-9079.07.patch,
> HDFS-9079.08.patch, HDFS-9079.09.patch, HDFS-9079.10.patch,
> HDFS-9079.11.patch, HDFS-9079.12.patch, HDFS-9079.13.patch,
> HDFS-9079.14.patch, HDFS-9079.15.patch
>
>
> A non-striped DataStreamer goes through the following steps in error handling:
> {code}
> 1) Finds error => 2) Asks NN for new GS => 3) Gets new GS from NN => 4)
> Applies new GS to DN (createBlockOutputStream) => 5) Ack from DN => 6)
> Updates block on NN
> {code}
> With multiple streamer threads run in parallel, we need to correctly handle a
> large number of possible combinations of interleaved thread events. For
> example, {{streamer_B}} starts step 2 in between events {{streamer_A.2}} and
> {{streamer_A.3}}.
> HDFS-9040 moves steps 1, 2, 3, 6 from streamer to {{DFSStripedOutputStream}}.
> This JIRA proposes some further optimizations based on HDFS-9040:
> # We can preallocate GS when NN creates a new striped block group
> ({{FSN#createNewBlock}}). For each new striped block group we can reserve
> {{NUM_PARITY_BLOCKS}} GS's. If more than {{NUM_PARITY_BLOCKS}} errors have
> happened we shouldn't try to further recover anyway.
> # We can use a dedicated event processor to offload the error handling logic
> from {{DFSStripedOutputStream}}, which is not a long running daemon.
> # We can limit the lifespan of a streamer to be a single block. A streamer
> ends either after finishing the current block or when encountering a DN
> failure.
> With the proposed change, a {{StripedDataStreamer}}'s flow becomes:
> {code}
> 1) Finds DN error => 2) Notify coordinator (async, not waiting for response)
> => terminates
> 1) Finds external error => 2) Applies new GS to DN (createBlockOutputStream)
> => 3) Ack from DN => 4) Notify coordinator (async, not waiting for response)
> {code}
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