How to create fast and efficient filestream writes on large sparse files

Question

I have an application that writes large files in multiple segments. I use FileStream.Seek to position each wirte. It appears that when I call FileStream.Write at a deep position in a sparse file the write triggers a "backfill" operation (writeing 0s) on all preceding bytes which is slow.

Is there a more efficient way of handling this situation?

The below code demonstrates the problem. The initial write takes about 370 MS on my machine.

    public void WriteToStream()
    {
        DateTime dt;
        using (FileStream fs = File.Create("C:\\testfile.file"))
        {   
            fs.SetLength(1024 * 1024 * 100);
            fs.Seek(-1, SeekOrigin.End);
            dt = DateTime.Now;
            fs.WriteByte(255);              
        }

        Console.WriteLine(@"WRITE MS: " + DateTime.Now.Subtract(dt).TotalMilliseconds.ToString());
    }

Answer 1

NTFS does support Sparse Files, however there is no way to do it in .net without p/invoking some native methods.

It is not very hard to mark a file as sparse, just know once a file is marked as a sparse file it can never be converted back in to a non sparse file except by coping the entire file in to a new non sparse file.

Example useage

class Program
{
    [DllImport("Kernel32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    private static extern bool DeviceIoControl(
        SafeFileHandle hDevice,
        int dwIoControlCode,
        IntPtr InBuffer,
        int nInBufferSize,
        IntPtr OutBuffer,
        int nOutBufferSize,
        ref int pBytesReturned,
        [In] ref NativeOverlapped lpOverlapped
    );

    static void MarkAsSparseFile(SafeFileHandle fileHandle)
    {
        int bytesReturned = 0;
        NativeOverlapped lpOverlapped = new NativeOverlapped();
        bool result =
            DeviceIoControl(
                fileHandle,
                590020, //FSCTL_SET_SPARSE,
                IntPtr.Zero,
                0,
                IntPtr.Zero,
                0,
                ref bytesReturned,
                ref lpOverlapped);
        if(result == false)
            throw new Win32Exception();
    }

    static void Main()
    {
        //Use stopwatch when benchmarking, not DateTime
        Stopwatch stopwatch = new Stopwatch();

        stopwatch.Start();
        using (FileStream fs = File.Create(@"e:\Test\test.dat"))
        {
            MarkAsSparseFile(fs.SafeFileHandle);

            fs.SetLength(1024 * 1024 * 100);
            fs.Seek(-1, SeekOrigin.End);
            fs.WriteByte(255);
        }
        stopwatch.Stop();

        //Returns 2 for sparse files and 1127 for non sparse
        Console.WriteLine(@"WRITE MS: " + stopwatch.ElapsedMilliseconds); 
    }
}

---

Once a file has been marked as sparse it now behaves like you excepted it to behave in the comments too. You don't need to write a byte to mark a file to a set size.

static void Main()
{
    string filename = @"e:\Test\test.dat";

    using (FileStream fs = new FileStream(filename, FileMode.Create))
    {
        MarkAsSparseFile(fs.SafeFileHandle);

        fs.SetLength(1024 * 1024 * 25);
    }
}