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https://issues.apache.org/jira/browse/LUCENE-1743?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12731638#action_12731638
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Uwe Schindler commented on LUCENE-1743:
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A typical example, where MMap would be the wrong thing are e.g. norms. They are
read one time and then the file is not accessed anymore. It would only be cool,
if MMapDir could directly return the mapped array (but MappedByteBuffer.array()
does not work) and use it as norms array. That would be cool.
My problem was more with all these small files like segments_XXXX and
segments.gen or *.del files. They are small and only used one time. Mapping
them is just waste of resources and completely useless (even slower that
opening them directly). This is why I said, some limit or file extension based
mapping would be good.
> MMapDirectory should only mmap large files, small files should be opened
> using SimpleFS/NIOFS
> ---------------------------------------------------------------------------------------------
>
> Key: LUCENE-1743
> URL: https://issues.apache.org/jira/browse/LUCENE-1743
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Store
> Affects Versions: 2.9
> Reporter: Uwe Schindler
> Assignee: Uwe Schindler
> Fix For: 3.1
>
>
> This is a followup to LUCENE-1741:
> Javadocs state (in FileChannel#map): "For most operating systems, mapping a
> file into memory is more expensive than reading or writing a few tens of
> kilobytes of data via the usual read and write methods. From the standpoint
> of performance it is generally only worth mapping relatively large files into
> memory."
> MMapDirectory should get a user-configureable size parameter that is a lower
> limit for mmapping files. All files with a size<limit should be opened using
> a conventional IndexInput from SimpleFS or NIO (another configuration option
> for the fallback?).
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