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Date: Wed, 22 Apr 2015 00:36:59 +0000 (UTC)
From: "Zhe Zhang (JIRA)" <jira@apache.org>
To: common-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HADOOP-11847) Enhance raw coder allowing to
 read least required inputs in decoding
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    [ https://issues.apache.org/jira/browse/HADOOP-11847?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14506111#comment-14506111 ] 

Zhe Zhang commented on HADOOP-11847:
------------------------------------

Thanks Kai for the patch. Please find my review below:
# We try to decode all null slots in the input arrays. I'm not sure if this will cause unnecessary computation. 
# Could you explain this change? Shouldn't the first argument be {{numDataUnits}}?
{code}
-      xorRawDecoder.initialize(getNumDataUnits(), 1, getChunkSize());
+      xorRawDecoder.initialize(getNumDataUnits() + getNumParityUnits() - 1,
+          1, getChunkSize());
{code}
# {{checkParameters}} goes through the input arrays once, and the {{badCount}} makes another pass. Can we just assert {{badCount + erasedIndexes.length == numDataUnits}}? 
# {{ensureWhenUseXXX}} needs some Javadoc. Maybe also add a better explanation than {{// Lazy on demand}}?
# These variable names look confusing: {{decodingDirectBufferInputs}} vs. {{decodingDirectBuffersForInput}}, and {{decodingDirectBufferOutputs}} vs. {{decodingDirectBuffersForOutput}}
# Is {{decodingByteArrayBuffersForInput}} always filled with zero bytes? I don't see where it's filled with actual data

> Enhance raw coder allowing to read least required inputs in decoding
> --------------------------------------------------------------------
>
>                 Key: HADOOP-11847
>                 URL: https://issues.apache.org/jira/browse/HADOOP-11847
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: io
>            Reporter: Kai Zheng
>            Assignee: Kai Zheng
>         Attachments: HADOOP-11847-v1.patch
>
>
> This is to enhance raw erasure coder to allow only reading least required inputs while decoding. It will also refine and document the relevant APIs for better understanding and usage. When using least required inputs, it may add computating overhead but will possiblly outperform overall since less network traffic and disk IO are involved.
> This is something planned to do but just got reminded by [~zhz]' s question raised in HDFS-7678, also copied here:
> bq.Kai Zheng I have a question about decoding: in a (6+3) schema, if block #2 is missing, and I want to repair it with blocks 0, 1, 3, 4, 5, 8, how should I construct the inputs to RawErasureDecoder#decode?
> With this work, hopefully the answer to above question would be obvious.


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