Java NIO FileChannel versus FileOutputstream performance / usefulness

My experience with larger files sizes has been that java.nio is faster than java.io. Solidly faster. Like in the >250% range. That said, I am eliminating obvious bottlenecks, which I suggest your micro-benchmark might suffer from. Potential areas for investigating:

The buffer size. The algorithm you basically have is

• copy from disk to buffer
• copy from buffer to disk

My own experience has been that this buffer size is ripe for tuning. I've settled on 4KB for one part of my application, 256KB for another. I suspect your code is suffering with such a large buffer. Run some benchmarks with buffers of 1KB, 2KB, 4KB, 8KB, 16KB, 32KB and 64KB to prove it to yourself.

Don't perform java benchmarks that read and write to the same disk.

If you do, then you are really benchmarking the disk, and not Java. I would also suggest that if your CPU is not busy, then you are probably experiencing some other bottleneck.

Don't use a buffer if you don't need to.

Why copy to memory if your target is another disk or a NIC? With larger files, the latency incured is non-trivial.

Like other have said, use FileChannel.transferTo() or FileChannel.transferFrom(). The key advantage here is that the JVM uses the OS's access to DMA (Direct Memory Access), if present. (This is implementation dependent, but modern Sun and IBM versions on general purpose CPUs are good to go.) What happens is the data goes straight to/from disc, to the bus, and then to the destination... bypassing any circuit through RAM or the CPU.

The web app I spent my days and night working on is very IO heavy. I've done micro benchmarks and real-world benchmarks too. And the results are up on my blog, have a look-see:

• Real world performance metrics: java.io vs. java.nio
• Real world performance metrics: java.io vs. java.nio (The Sequel)

Use production data and environments

Micro-benchmarks are prone to distortion. If you can, make the effort to gather data from exactly what you plan to do, with the load you expect, on the hardware you expect.

My benchmarks are solid and reliable because they took place on a production system, a beefy system, a system under load, gathered in logs. Not my notebook's 7200 RPM 2.5" SATA drive while I watched intensely as the JVM work my hard disc.

What are you running on? It matters.

If the thing you want to compare is performance of file copying, then for the channel test you should do this instead:

final FileInputStream inputStream = new FileInputStream(src);final FileOutputStream outputStream = new FileOutputStream(dest);final FileChannel inChannel = inputStream.getChannel();final FileChannel outChannel = outputStream.getChannel();inChannel.transferTo(0, inChannel.size(), outChannel);inChannel.close();outChannel.close();inputStream.close();outputStream.close();

This won't be slower than buffering yourself from one channel to the other, and will potentially be massively faster. According to the Javadocs:

Many operating systems can transfer bytes directly from the filesystem cache to the target channel without actually copying them.

Based on my tests (Win7 64bit, 6GB RAM, Java6), NIO transferFrom is fast only with small files and becomes very slow on larger files. NIO databuffer flip always outperforms standard IO.

• Copying 1000x2MB

  1. NIO (transferFrom) ~2300ms
  2. NIO (direct datababuffer 5000b flip) ~3500ms
  3. Standard IO (buffer 5000b) ~6000ms

• Copying 100x20mb

  1. NIO (direct datababuffer 5000b flip) ~4000ms
  2. NIO (transferFrom) ~5000ms
  3. Standard IO (buffer 5000b) ~6500ms

• Copying 1x1000mb

  1. NIO (direct datababuffer 5000b flip) ~4500s
  2. Standard IO (buffer 5000b) ~7000ms
  3. NIO (transferFrom) ~8000ms

The transferTo() method works on chunks of a file; wasn't intended as a high-level file copy method:How to copy a large file in Windows XP?