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https://issues.apache.org/jira/browse/LUCENE-4864?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13609584#comment-13609584
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Uwe Schindler commented on LUCENE-4864:
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Mike, the comparison should be between SimpleFSDir (which is the default on
windows for 32 bit), NIOFSDir and this new async one. It is of course slower
than MMap, that was expected. It is important to have hight concurrency.
> Add AsyncFSDirectory to work around Windows issues with NIOFS (Lucene 5.0
> only)
> -------------------------------------------------------------------------------
>
> Key: LUCENE-4864
> URL: https://issues.apache.org/jira/browse/LUCENE-4864
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/store
> Affects Versions: 5.0
> Reporter: Michael Poindexter
> Attachments: LUCENE-4864.patch, LUCENE-4864.patch
>
>
> On LUCENE-4848 a new directory implementation was proposed that uses
> AsyncFileChannel to make a sync-less directory implementation (only needed
> for IndexInput). The problem on Windows is that positional reads are
> impossible without overlapping (async) I/O, so FileChannel in the JDK has to
> syncronize all reads, because they consist of an atomic seek and atomic read.
> AsyncFSDirectoty would not have this issue, but has to take care of thread
> management, because you need a separate thread to get notified when the read
> is done. This involves overhead, but might still be better than the
> synchronization.
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