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https://issues.apache.org/jira/browse/HDFS-4697?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Colin Patrick McCabe resolved HDFS-4697.
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Resolution: Fixed
Fix Version/s: 2.3.0
Short-circuit now honors {{dfs.datanode.readahead.bytes}}, since HDFS-5634.
However, if you want short-circuit the full 4 MB of readahead that the datanode
is doing, you'll need to set {{dfs.client.read.shortcircuit.buffer.size}} to 4
megabytes in your configuration. Its value is specified in bytes and it
defaults to 1 megabyte. The tradeoff is slightly increased memory consumption.
> short-circuit reads do not honor readahead settings
> ---------------------------------------------------
>
> Key: HDFS-4697
> URL: https://issues.apache.org/jira/browse/HDFS-4697
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs-client
> Affects Versions: 2.0.3-alpha
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Priority: Minor
> Fix For: 2.3.0
>
>
> Neither the new nor the legacy short-circuit read implementations honor
> {{dfs.datanode.readahead.bytes}}. This can result in scenarios where
> non-short-circuit reads are faster for long reads, simply because they are
> doing more readahead, and SCR is not. We should do readahead in both cases
> if it is configured.
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