File copy-on-write in Java?
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=开发者_运维问答auto
in some library that hopefully exists and somebody has heard of, so they can tell me? :-)
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:
- allocate a direct buffer,
- populate its parameters (being careful to properly deal with machine endianness); you'll need to open FDs as well.
- get a pointer with
Native.getDirectBufferPointer
- Invoke the ioctl with this.
- 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.
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