FileChannelJournalSegmentReader is using FileChannel methods in rather sub-optimal ways, such as:

        long position = channel.position() + memory.position();
        channel.position(position);
        memory.clear();
        channel.read(memory);
        channel.position(position);
        memory.flip();

While memory.position() is HeapByteBuffer's local field, not shared with anyone else, channel.position() is kernel-side status tied to the channel and hence shared between all users of that channel.

That is problematic for three reasons:

• each call to get or set the position ends up being a system call
• every call needs to be made with FileChannel.positionLock held
• it might be shared resource, hence we set it explicitly before each read – because it may have been set by a different reader, or the writer

All of this dance is done because we do not take advantage of FileChannel API: it offers a read(ByteBuffer, long) method, where the second argument is the desired start position.

Reorganize the code to use that method and maintain the position in a field (IF we really need that).

This will cut down on system calls, as each read will be a single syscall, remove the locking overhead (except on Windows) and, in future, allow us to share a single FileChannel.