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?
Improvement
Closed
Minor
CompoundFileWriter should pre-set its file length
