Copy raw data of block device using Java

Question

I have 2 disks in the Linux system, say /dev/dsk1 and /dev/dsk2 , and I'm trying to read the raw data from dsk1 in bytes and write them into dsk2 , in order to make dsk2 an exact copy of dsk1 . I tried to do that in the following way (executed with sudo ):

import...

public class Main {
    public static void main(String[] args) throws NoSuchAlgorithmException, IOException {
        Path src = new File("/dev/dsk1").toPath();
        Path dst = new File("/dev/dsk2").toPath();
        FileChannel r = FileChannel.open(src, StandardOpenOption.READ, StandardOpenOption.WRITE);
        FileChannel w = FileChannel.open(dst, StandardOpenOption.READ, StandardOpenOption.WRITE);
        long size = r.size();
        ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
        for (int offset = 0; offset < size; offset+=1024) {
            r.position(offset);
            w.position(offset);
            r.read(byteBuffer);
            byteBuffer.flip();
            w.write(byteBuffer);
            byteBuffer.clear();
        }
        r.close();
        w.close();
    }
}

but after writing all the bytes in dsk1 to dsk2 , dsk2 's filesystem seems to be corrupted. No files can be found in it and if I try to mkdir it will say "structure needs cleaning".

I've tested the above code on regular files, like a text1.txt containing a few characters as src and an empty text2.txt as dst , and it worked fine.

Did I miss something there when reading & writing raw data on block device?

Answer 1

You never check if read method read all 1024 bytes, or if write method wrote them all. Most likely you're leaving gaps in the copy.

There's no magic involved reading from and writing to devices. The first thing I would try is this:

try (FileInputStream src = new FileInputStream("/dev/dsk1");
     FileOutputStream dst = new FileOutputStream("/dev/dsk2")) {
    src.transferTo(dst);
}