RFR: 8264744: (fs) Use file cloning in Linux version of Files::copy method [v13]

[Mike Hearn]

On Tue, 6 Sep 2022 17:20:59 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> Add file cloning to `java.nio.channels.FileChannel::transferTo` and `java.nio.file.Files.copy(Path,Path)`.
>
> Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8264744: Remove search for CoW volumes; run same directory tests on test.dir if it has a different format

Thank you! I'm the one who filed the original webbug report (some day I must figure out how to get direct access to the JDK bug tracker). 

This wasn't a theoretical request, it was motivated by a real product that my company has just launched called [Conveyor](https://conveyor.hydraulic.dev/). Conveyor is a sort of jpackage++ and is structured as a parallel build system in which intermediate steps are cached. The different tasks sometimes have to copy large files around (like JDKs, JARs etc) and using kernel level COW cloning will yield a large boost to performance on filesystems that support it.

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PR: https://git.openjdk.org/jdk/pull/9486