RFR: 6980847: (fs) Files.copy needs to be tuned

[Roger Riggs]

On Wed, 15 Jun 2022 00:20:58 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

> Modify `UnixCopyFile.copyFile()` to set the transfer size to the least common multiple of the source and destination file block sizes if the block sizes are not equal. Modify `WindowsFileCopy.copy()` to set `COPY_FILE_NO_BUFFERING` in the flags passed to `CopyFileEx()` if the size of the transfer is greater than a threshold.

src/java.base/unix/classes/sun/nio/fs/UnixCopyFile.java line 232:

> 230: 
> 231:         while (u != v) {
> 232:             if (u < v)

It might be worth a comment about the expected inputs so it is known to converge quickly.
In the general case it might wrap around and go unstable.

src/java.base/unix/classes/sun/nio/fs/UnixCopyFile.java line 664:

> 662:     // -- native methods --
> 663: 
> 664:     static native long transferSize0();

If the `transferSize` is fixed, there seems to be little reason to call native to retrieve it.
(Perhaps hoping for the OS to provide it.)

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PR: https://git.openjdk.org/jdk/pull/9161