File copy-on-write in Java?

Question

I'm using brtfs, and I'd like my Java program to copy a set of large files copy-on-write.

In other words, what's the equivalent of cp --reflink=auto in some library that hopefully exists and somebody has heard of, so they can tell me? :-)

Answer 1

There isn't a Java-specific API to my knowledge, because this is a rather specific, OS-dependent, and filesystem-dependent feature. However, with the help of a library that can issue ioctls (e.g. this one which I have no affiliation with and found by googling), you can issue the ficlonerange ioctl.

To invoke it you'll need to put together a struct:

           struct file_clone_range {
               __s64 src_fd;
               __u64 src_offset;
               __u64 src_length;
               __u64 dest_offset;
           };

It's a bit roundabout in Java, but as an example, you should be able to do this as follows using the linked library:

1. allocate a direct buffer,
2. populate its parameters (being careful to properly deal with machine endianness); you'll need to open FDs as well.
3. get a pointer with Native.getDirectBufferPointer
4. Invoke the ioctl with this.
5. error-check the result and take appropriate action if the ioctl fails or is not supported.

If that seems too brittle, consider writing a C or C++ library that calls the ioctl and has a more convenient API, and then call into it via JNI.