How to Copy a File Without Using the Windows File Cache

When it comes to file management within Windows, you might encounter situations where the operating system’s file caching can hinder performance—especially when dealing with large files. For instance, copying a large file from a USB drive or a server to your local machine can be problematic as Windows caches the file, which might lead to swapping out data and increase the time it takes for the operation to complete.

In this blog post, we’ll explore how to address this issue by copying files without making use of the Windows file system cache, specifically using C#.

Understanding the Windows File System Cache

Before diving into the coding solution, it’s important to understand what the file system cache is. Windows uses a cache to speed up file operations, which typically involves:

• Storing file data in memory for quicker access.
• Looking to offload data from slower devices like USB drives onto the faster system memory.

However, with larger files (think 2 GiB or more), this caching process can lead to inefficiencies, as the system tries to manage its available memory.

The Solution: Using Win32 API Calls

To bypass this caching behavior, Windows provides specific flags that you can use when working with file operations. These flags are:

• FILE_FLAG_WRITE_THROUGH: This flag ensures that the writes to the file are directly written out to the storage device and are not cached in memory.
• FILE_FLAG_NO_BUFFERING: This flag disables the system’s cache for the file, meaning all read and write operations go directly to the disk without being cached.

You can utilize these flags in your C# code to achieve direct file copying without using the cache.

Step-by-Step Implementation in C#

Here’s how to implement the solution in C#:

1. Add Necessary Libraries: Ensure you include the required namespaces for file operations and interop services.

   using System;
   using System.IO;
   using System.Runtime.InteropServices;
   

2. Declare Native Methods: You need to declare the necessary Win32 API functions.

   [DllImport("kernel32.dll", SetLastError = true)]
   public static extern IntPtr CreateFile(
       string lpFileName,
       uint dwDesiredAccess,
       uint dwShareMode,
       IntPtr lpSecurityAttributes,
       uint dwCreationDisposition,
       uint dwFlagsAndAttributes,
       IntPtr hTemplateFile);
   
   [DllImport("kernel32.dll", SetLastError = true)]
   public static extern bool ReadFile(
       IntPtr hFile,
       byte[] lpBuffer,
       uint nNumberOfBytesToRead,
       out uint lpNumberOfBytesRead,
       IntPtr lpOverlapped);
   
   [DllImport("kernel32.dll", SetLastError = true)]
   public static extern bool WriteFile(
       IntPtr hFile,
       byte[] lpBuffer,
       uint nNumberOfBytesToWrite,
       out uint lpNumberOfBytesWritten,
       IntPtr lpOverlapped);
   
   [DllImport("kernel32.dll", SetLastError = true)]
   public static extern bool CloseHandle(IntPtr hObject);
   

3. Using the Flags: In your file copy function, use the flags to create the file handles.

   public void CopyFileWithoutCache(string sourceFilePath, string destFilePath)
   {
       const uint GENERIC_READ = 0x80000000;
       const uint GENERIC_WRITE = 0x40000000;
       const uint OPEN_EXISTING = 3;
       const uint FILE_FLAG_WRITE_THROUGH = 0x80000000;
       const uint FILE_FLAG_NO_BUFFERING = 0x20000000;
   
       IntPtr sourceHandle = CreateFile(sourceFilePath, GENERIC_READ, 0, IntPtr.Zero, OPEN_EXISTING, FILE_FLAG_WRITE_THROUGH | FILE_FLAG_NO_BUFFERING, IntPtr.Zero);
       IntPtr destHandle = CreateFile(destFilePath, GENERIC_WRITE, 0, IntPtr.Zero, OPEN_EXISTING, FILE_FLAG_WRITE_THROUGH | FILE_FLAG_NO_BUFFERING, IntPtr.Zero);
   
       // Implement file reading and writing logic here...
   
       CloseHandle(sourceHandle);
       CloseHandle(destHandle);
   }
   

Conclusion

With the steps outlined above, you can efficiently copy large files without the interference of the Windows file system cache. This method is particularly beneficial when dealing with high-capacity files transferred from slow devices like USB drives.

For more details, check out the Microsoft documentation on the FILE_FLAG_WRITE_THROUGH and FILE_FLAG_NO_BUFFERING.

By understanding and leveraging these flags, you can improve the performance of your file operations significantly. Happy coding!