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https://issues.apache.org/jira/browse/HDFS-4953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13720384#comment-13720384
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Tsuyoshi OZAWA commented on HDFS-4953:
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This work is very interesting to increase the throughput of HDFS. IMHO, more
benchmarks are needed to clarify when to use this feature. For instance:
1. Read large objects which doesn't fit in memory.
2. Basic benchmarks on CentOS 5. Old linux kernels cause page faults with the
giant lock when user program touches virtual memory which is mapped to a file
on disk by mmap(2). This can be problem for CDH users, because CDH supports
CentOS 5[1].
[1] http://lwn.net/Articles/200215/
> enable HDFS local reads via mmap
> --------------------------------
>
> Key: HDFS-4953
> URL: https://issues.apache.org/jira/browse/HDFS-4953
> Project: Hadoop HDFS
> Issue Type: New Feature
> Affects Versions: 2.3.0
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Attachments: benchmark.png, HDFS-4953.001.patch, HDFS-4953.002.patch,
> HDFS-4953.003.patch, HDFS-4953.004.patch, HDFS-4953.005.patch
>
>
> Currently, the short-circuit local read pathway allows HDFS clients to access
> files directly without going through the DataNode. However, all of these
> reads involve a copy at the operating system level, since they rely on the
> read() / pread() / etc family of kernel interfaces.
> We would like to enable HDFS to read local files via mmap. This would enable
> truly zero-copy reads.
> In the initial implementation, zero-copy reads will only be performed when
> checksums were disabled. Later, we can use the DataNode's cache awareness to
> only perform zero-copy reads when we know that checksum has already been
> verified.
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