Question

I’m trying to determine worst case disk speed, so I wrote the following function.

static public decimal MBytesPerSec(string volume)
{
    string filename = volume + "\\writetest.tmp";

    if (System.IO.File.Exists(filename))
        System.IO.File.Delete(filename);

    System.IO.StreamWriter file = new System.IO.StreamWriter(filename);

    char[] data = new char[64000];
    Stopwatch watch = new Stopwatch();
    watch.Start();

    int i = 0;

    for (; i < 1000; i++)
    {
        file.Write(data);
        if (watch.ElapsedMilliseconds > 2000)
        {
            break;
        }
    }

    watch.Stop();
    file.Close();

    System.IO.File.Delete(volume + "\\test.txt");
    decimal mbytessec = (i * 64 / watch.ElapsedMilliseconds);
    return mbytessec;
}

The function works OK, but the writes are getting cached, so the speed is not worst case.

In WIN32 C++, I would simply create the file with FILE_FLAG_NO_BUFFERING, FILE_FLAG_WRITE_THROUGH options, and then make sure to follow the non-cached writing rules (write to the file at sector size offsets, with minimum of 4k writes)

I found one article that discusses the .NET technique.

So I wrote a new function (ignore the math errors).

static public decimal MBytesPerSecNonCached(string volume)
{
    const FileOptions FILE_FLAG_NO_BUFFERING = (FileOptions)0x20000000;

    string filename = volume + "\\writetest.tmp";

    using (FileStream fs = new FileStream(filename, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None, 1024, FileOptions.WriteThrough | FILE_FLAG_NO_BUFFERING))
    {
        byte[] data = new byte[65535];
        int i = 0;

        Stopwatch watch = new Stopwatch();
        watch.Start();

        for (; i < 1000; i++)
        {
            fs.Write(data, 0, 65535);
            if (watch.ElapsedMilliseconds > 2000)
            {
                break;
            }
        }

        watch.Stop();
        fs.Close();

        System.IO.File.Delete(filename);

        decimal mbytessec = (i * 64 / watch.ElapsedMilliseconds);

        return mbytessec;
    }
}

This function works for 4k, 16K and 32K write sizes, but once I try 64K write sizes, I get an exception:

IO operation will not work. Most likely the file will become too long or the handle was not opened to support synchronous IO operations.

So, how can I fix this so I can test with larger than 32KB write sizes (64KB to 4096KB)?

Answer 1

Try some unmanaged code:

[DllImport("kernel32", SetLastError = true)]
        static extern unsafe SafeFileHandle CreateFile(
            string FileName,           // file name
            uint DesiredAccess,        // access mode
            uint ShareMode,            // share mode
            IntPtr SecurityAttributes, // Security Attr
            uint CreationDisposition,  // how to create
            uint FlagsAndAttributes,   // file attributes
            IntPtr hTemplate // template file  
            );
const uint FILE_FLAG_NO_BUFFERING = 0x20000000;

SafeFileHandle handle = CreateFile("filename",
                            (uint)FileAccess.Write,
                            (uint)FileShare.None,
                            IntPtr.Zero,
                            (uint)FileMode.Open,
                             FILE_FLAG_NO_BUFFERING,
                            IntPtr.Zero);

var unBufferedStream = new FileStream(handle,FileAccess.Read,blockSize,false);

now you should have access to an unbuffered stream which you can read and write however you please with no constraints

For the record….you can also disable caching like this:

[DllImport("KERNEL32", SetLastError = true)]
        public extern static int DeviceIoControl(IntPtr hDevice, uint IoControlCode,
            IntPtr lpInBuffer, uint InBufferSize,
            IntPtr lpOutBuffer, uint nOutBufferSize,
            ref uint lpBytesReturned,
            IntPtr lpOverlapped);
        [DllImport("KERNEL32", SetLastError = true)]
        public extern static int CloseHandle(
        IntPtr hObject);

[StructLayout(LayoutKind.Sequential)]
        public struct DISK_CACHE_INFORMATION
        {            
            public byte ParametersSavable;            
            public byte ReadCacheEnabled;            
            public byte WriteCacheEnabled;
            public int ReadRetentionPriority;//DISK_CACHE_RETENTION_PRIORITY = enum = int
            public int WriteRetentionPriority;//DISK_CACHE_RETENTION_PRIORITY = enum = int
            public Int16 DisablePrefetchTransferLength;//WORD            
            public byte PrefetchScalar;            
        }

public void SetDiskCache(byte val)
        {
            IntPtr h = CreateFile("\\\\.\\PHYSICALDRIVE0", (uint)FileAccess.Read | (uint)FileAccess.Write, (uint)FileShare.Write, IntPtr.Zero, (uint)FileMode.Open, 0, IntPtr.Zero);
            DISK_CACHE_INFORMATION sInfo = new DISK_CACHE_INFORMATION();
            IntPtr ptrout = Marshal.AllocHGlobal(Marshal.SizeOf(sInfo));
            Marshal.StructureToPtr(sInfo, ptrout, true);            
            uint dwWritten = 0;
            int ret = DeviceIoControl(h,IOCTL_DISK_GET_CACHE_INFORMATION,IntPtr.Zero,0,ptrout,(uint)Marshal.SizeOf(sInfo),ref dwWritten,IntPtr.Zero);            
            sInfo = (DISK_CACHE_INFORMATION)Marshal.PtrToStructure(ptrout,typeof(DISK_CACHE_INFORMATION));            
            sInfo.ReadCacheEnabled = val;
            // acuma trimite structura modificata
            IntPtr ptrin = Marshal.AllocHGlobal(Marshal.SizeOf(sInfo));
            Marshal.StructureToPtr(sInfo, ptrin, true);            
            ret = DeviceIoControl(h, IOCTL_DISK_SET_CACHE_INFORMATION, ptrin, (uint)Marshal.SizeOf(sInfo), IntPtr.Zero, 0, ref dwWritten, IntPtr.Zero);            
            CloseHandle(h);            
        }