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https://issues.apache.org/jira/browse/LUCENE-1743?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12731639#action_12731639
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Earwin Burrfoot commented on LUCENE-1743:
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bq. 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.
I can only reiterate my point. These files aren't opened like 10k files per 
second, so your win is going to be in the order of microseconds per reopen - at 
the cost of increased complexity.

> 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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