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From "Jing Zhao (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8319) Erasure Coding: support decoding for stateful read
Date Thu, 04 Jun 2015 00:35:38 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8319?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14571900#comment-14571900
] 

Jing Zhao commented on HDFS-8319:
---------------------------------

bq. Using the same buffer type (no mixing) will also make the buffer allocation and management
easier overall. I'm wondering could we avoid the mixing?

Yes, we can avoid this by always allocating direct buffer for parity blocks as well. But unlike
the buffer used by data blocks, this (64KB * 3) buffer may never be used if decoding is unnecessary.


More importantly, from the decoder API point of view, there is no API level restriction to
prevent the mixing util we hit exception during the decoding. I would prefer the API to be
more clear and friendly. And before we have this improvement, I think we need the direct buffer
detection change to avoid exception in mixing scenarios.

bq. Did you see any problem doing that way?
Yes, the current decode function directly assign array() to newInputs, which means newInputs
take the complete underlying byte array, instead of the slices. This lead to wrong the decoding
result in my test.
{code}
    ByteBuffer buffer;
    for (int i = 0; i < inputs.length; ++i) {
      buffer = inputs[i];
      if (buffer != null) {
        inputOffsets[i] = buffer.position();
        newInputs[i] = buffer.array();
      }
    }
{code}

> Erasure Coding: support decoding for stateful read
> --------------------------------------------------
>
>                 Key: HDFS-8319
>                 URL: https://issues.apache.org/jira/browse/HDFS-8319
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Jing Zhao
>            Assignee: Jing Zhao
>         Attachments: HDFS-8319.001.patch, HDFS-8319.002.patch, HDFS-8319.003.patch
>
>
> HDFS-7678 adds the decoding functionality for pread. This jira plans to add decoding
to stateful read.



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