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https://issues.apache.org/jira/browse/HDFS-6450?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14349499#comment-14349499
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Zhe Zhang commented on HDFS-6450:
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In the erasure coding project we need to implement the logic of reading from a
striped file (HDFS-7782). Hedged reading is a natural fit to support striping
layout, because each I/O is likely to cover multiple DNs. So I'd like to dig a
little deeper on this JIRA to see if hedged non-positional read should be added
to DFIS or only the subclass for erasure coding.
bq. Yeah... if the first read, done with the old block reader, doesn't finish
first, then I think we should forget about that block reader... even if it's a
BlockReaderLocal and the new one is remote. After all, slow or misbehaving
local disks are one of the main problems we're trying to cover up with hedged
reads. As you pointed out, we need to close the old block reader in this
situation.
[~cmccabe] I like the idea of reusing the {{blockReader}} at first Future, and
updating it to the winning reader. The only concern is we'll never come back to
the local reader if it goes slow once. Should we give it another chance?
> Support non-positional hedged reads in HDFS
> -------------------------------------------
>
> Key: HDFS-6450
> URL: https://issues.apache.org/jira/browse/HDFS-6450
> Project: Hadoop HDFS
> Issue Type: Improvement
> Affects Versions: 2.4.0
> Reporter: Colin Patrick McCabe
> Assignee: Liang Xie
> Attachments: HDFS-6450-like-pread.txt
>
>
> HDFS-5776 added support for hedged positional reads. We should also support
> hedged non-position reads (aka regular reads).
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