Question

I have a piece of code that needs to be able to modify a few bytes towards the end of a file. The problem is that the files are huge. Up to 100+ Gb.

I need the operation to be as fast as possible but after hours of Googeling, it looks like .Net is rather limited here???

I have mostly been trying using System.IO.FileStream and know of no other methods. A “reverse” filestream would do but I have know idea how to create one (write from the end instead of the beginning).

Here is sort of what I do: (Note: the time is spent when closing the stream)

    static void Main(string[] args)
    {    
        //Simulate a large file
        int size = 1000 * 1024 * 1024;
        string filename = "blah.dat";
        FileStream fs = new FileStream(filename, FileMode.Create);
        fs.SetLength(size);
        fs.Close();

        //Modify the last byte
        fs = new FileStream(filename, FileMode.Open);

        //If I don't seek, the modification happens instantly
        fs.Seek(-1, SeekOrigin.End);
        fs.WriteByte(255);

        //Now, since I am modifying the last byte, 
        //this last step is very slow
        fs.Close();
    }
}

Answer 1

Like Darin already noted, this is an artifact of your ‘simulation’ of a large file.

The delay is from actually ‘filling up’ the file, the delay only happens the first time. If you repeat the part from //Modify the last byte to fs.Close(); it will be very fast.