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https://issues.apache.org/jira/browse/LUCENE-705?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12564355#action_12564355
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Michael McCandless commented on LUCENE-705:
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{quote}
Does this mean one will no longer be able to tell exactly how large the index
really is (because some portion of some data files will actually be empty)?
{quote}
Only while the CFS is being built. After it's done being built, then
it is "fully occupied" (no portion are empty).
> CompoundFileWriter should pre-set its file length
> -------------------------------------------------
>
> Key: LUCENE-705
> URL: https://issues.apache.org/jira/browse/LUCENE-705
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Affects Versions: 2.1
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Priority: Minor
> Fix For: 2.4
>
> Attachments: LUCENE-705.patch
>
>
> I've read that if you are writing a large file, it's best to pre-set
> the size of the file in advance before you write all of its contents.
> This in general minimizes fragmentation and improves IO performance
> against the file in the future.
> I think this makes sense (intuitively) but I haven't done any real
> performance testing to verify.
> Java has the java.io.File.setLength() method (since 1.2) for this.
> We can easily fix CompoundFileWriter to call setLength() on the file
> it's writing (and add setLength() method to IndexOutput). The
> CompoundFileWriter knows exactly how large its file will be.
> Another good thing is: if you are going run out of disk space, then,
> the setLength call should fail up front instead of failing when the
> compound file is actually written. This has two benefits: first, you
> find out sooner that you will run out of disk space, and, second, you
> don't fill up the disk down to 0 bytes left (always a frustrating
> experience!). Instead you leave what space was available
> and throw an IOException.
> My one hesitation here is: what if out there there exists a filesystem
> that can't handle this call, and it throws an IOException on that
> platform? But this is balanced against possible easy-win improvement
> in performance.
> Does anyone have any feedback / thoughts / experience relevant to
> this?
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