optimize disk cache io

[Kila2]

Utilize NIO instead of InputStream. For MacOS, NIO copy can employ fcopyfile for non-blocking IO copy to optimize IO performance.

[fmeum]

@tjgq Have there been any experiments at Google on replacing some of the custom filesystem functions with Java NIO? I recently used Files.move to replace the JavaIoFilesystem.renameTo implementation and wondered whether it would make sense to adopt it more generally.

[tjgq]

I am sympathetic to the objective of this PR (the sendfile/fcopyfile optimization in NIO definitely sounds worthwhile), but I'd like to do some benchmarking of my own before we accept it. That will probably take a while, as I'm on vacation next week.

[tjgq]

We need it unless you can convince me that the scenario described in the comment is not a concern (https://danluu.com/deconstruct-files/ is a good reference on what can go wrong when a system crashes in the middle of a filesystem operation).

[Kila2]

I looked at the NIO source code. Only public static Path copy(Path source, Path target, CopyOption... options) throws IOException can optimize the IO operation, and there is no fsync method available.

[matthewjh]

Any update here? @Kila2 @fmeum @tjgq