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https://issues.apache.org/jira/browse/HDFS-4953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13737430#comment-13737430
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Colin Patrick McCabe commented on HDFS-4953:
--------------------------------------------
HDFS-4949 is a related, but independent change. In HDFS-4949, the DataNode
manages regions of memory using {{mmap}} and {{mlock}}. The {{mmap}} on the
DataNode side is done only to make {{mlock}} possible. The {{mmap}} on the
client side is done to allow the client to read from the {{mmap}} rather than
using the {[read}} syscall. {{mmap}} regions are specific to a particular
process-- an {{mmap}} in the DataNode is of no use to the client process.
ClientMmapManager is named the way it is to make it obvious that it's a
*client* thing, not a DataNode thing.
> 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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