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https://issues.apache.org/jira/browse/HDFS-4953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13732613#comment-13732613
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Colin Patrick McCabe commented on HDFS-4953:
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bq. To remove the SUN package dependency: how about reusing the native buffer
for a different file when its ref-count is zero, rather than allocating and
deallocating each time?
You cannot reuse an mmap for one file on a different file.
bq. I didn't find the definition of struct hadoopZeroCopyCursor. Is any header
file missed in the patch?
We typecast {{struct hadoopZeroCopyCursor}} to a {{jobject}} whenever it is
used. Since we don't dereference the type, there is no need for a type
definition. Having the type is useful because it prevents callers from passing
arbitrary pointers to {{hadoopZeroCopyRead}} and other functions.
I don't have a strong feeling about it either way, but I would prefer to get
this into trunk, since I don't think there are any dependencies on HDFS-4949.
Since there are other outstanding JIRAs on {[BlockReaderLocal}}, it would be
nice to avoid conflicts.
> 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,
> HDFS-4953.006.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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