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https://issues.apache.org/jira/browse/HDFS-223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15263601#comment-15263601
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Duo Zhang commented on HDFS-223:
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In HBASE-14790 we have implemented a {{FanOutOneBlockAsyncDFSOutput}} based on
netty. It performs much better than the default {{DFSOutputStream}} for WAL in
HBase.
And we plan to move these stuffs into HDFS since it should belong to HDFS. Of
course this is not a simple copy-paste work, the implementation in HBase is not
suitable for general use.
Will come back later with a proposal on the asynchronous API first.
Thanks.
> Asynchronous IO Handling in Hadoop and HDFS
> -------------------------------------------
>
> Key: HDFS-223
> URL: https://issues.apache.org/jira/browse/HDFS-223
> Project: Hadoop HDFS
> Issue Type: New Feature
> Reporter: Raghu Angadi
> Attachments: GrizzlyEchoServer.patch, MinaEchoServer.patch
>
>
> I think Hadoop needs utilities or framework to make it simpler to deal with
> generic asynchronous IO in Hadoop.
> Example use case :
> Its been a long standing problem that DataNode takes too many threads for
> data transfers. Each write operation takes up 2 threads at each of the
> datanodes and each read operation takes one irrespective of how much activity
> is on the sockets. The kinds of load that HDFS serves has been expanding
> quite fast and HDFS should handle these varied loads better. If there is a
> framework for non-blocking IO, read and write pipeline state machines could
> be implemented with async events on a fixed number of threads.
> A generic utility is better since it could be used in other places like
> DFSClient. DFSClient currently creates 2 extra threads for each file it has
> open for writing.
> Initially I started writing a primitive "selector", then tried to see if such
> facility already exists. [Apache MINA|http://mina.apache.org] seemed to do
> exactly this. My impression after looking the the interface and examples is
> that it does not give kind control we might prefer or need. First use case I
> was thinking of implementing using MINA was to replace "response handlers" in
> DataNode. The response handlers are simpler since they don't involve disk
> I/O. I [asked on MINA user
> list|http://www.nabble.com/Async-events-with-existing-NIO-sockets.-td18640767.html],
> but looks like it can not be done, I think mainly because the sockets are
> already created.
> Essentially what I have in mind is similar to MINA, except that read and
> write of the sockets is done by the event handlers. The lowest layer
> essentially invokes selectors, invokes event handlers on single or on
> multiple threads. Each event handler is is expected to do some non-blocking
> work. We would of course have utility handler implementations that do read,
> write, accept etc, that are useful for simple processing.
> Sam Pullara mentioned that [xSockets|http://xsocket.sourceforge.net/] is more
> flexible. It is under GPL.
> Are there other such implementations we should look at?
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