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https://issues.apache.org/jira/browse/HDFS-4953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13760804#comment-13760804
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Colin Patrick McCabe commented on HDFS-4953:
--------------------------------------------
I've been thinking about this, and I think it might be possible to improve on
the current API.
Maybe all we need is something like this:
{code}
in DFSInputStream:
ZeroBuffer readZero(ByteBuffer fallback, int maxLength);
ZeroBuffer:
implements Closeable (for close)
implements eof() (returns true if there are no more bytes to read)
implements all ByteBuffer methods by forwarding them to the enclosed
ByteBuffer
{code}
This API would be implemented for every filesystem, not just HDFS.
The constraints here would be:
* maxLength >= 0
* you can't reuse a fallback buffer until you close the associated ZeroBuffer
(we can enforce this by throwing an exception in this case)
* ZeroBuffers are immutable once created-- until you call close on them.
This gets rid of a few of the awkward issues with the current API, which I
think are:
* the current API requires users to special-case HDFS (since other FSes throw
ZeroCopyUnavailableException)
* the current API shares the file position between the cursors and the stream,
which is unintuitive.
* the current API puts the read call inside the cursor object, which is
different than the other read methods.
> 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
> Fix For: HDFS-4949
>
> 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, HDFS-4953.007.patch, HDFS-4953.008.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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