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From "Zhe Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9079) Erasure coding: preallocate multiple generation stamps and serialize updates from data streamers
Date Mon, 28 Aug 2017 17:30:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9079?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16144094#comment-16144094

Zhe Zhang commented on HDFS-9079:

Unassigning myself from the JIRA as I won't be able to work on it in the next couple of months.
[~andrew.wang] [~drankye] If you see anyone with some cycles working on this that'd be great.
I'm still a little concerned about the complexity on the write pipeline of EC, and I think
this will help.

> Erasure coding: preallocate multiple generation stamps and serialize updates from data
> ------------------------------------------------------------------------------------------------
>                 Key: HDFS-9079
>                 URL: https://issues.apache.org/jira/browse/HDFS-9079
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: erasure-coding
>    Affects Versions: HDFS-7285
>            Reporter: Zhe Zhang
>              Labels: hdfs-ec-3.0-nice-to-have
>         Attachments: 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, HDFS-9079-HDFS-7285.00.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) =>
> 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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