Windows NT Contains File System Tunneling Capabilities

IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base: The Microsoft Windowsproducts listed at the beginning of this artice contain file system tunneling capabilities to enable compatibility with programs that rely on file systems being able to hold onto file meta-info for a short period of time. This occurs after deletion or renaming and re-introducing a new directory entry with that meta-info (if a create or rename occurs to cause a file of that name to appear again in a short period of time).

When a name is removed from a directory (rename or delete), its short/long name pair and creation time are saved in a cache, keyed by the name that was removed. When a name is added to a directory (rename or create), the cache is searched to see if there is information to restore. The cache is effective per instance of a directory. If a directory is deleted, the cache for it is removed.

These paired operations can cause tunneling on "name."

• delete(name)/create(name)
  
  
• delete(name)/rename(source, name)
  
  
• rename(name, newname)/create(name)
  
  
• rename(name, newname)/rename(source, name)

The idea is to mimic the behavior MS-DOS programs expect when they use the safe save method. They copy the modified data to a temporary file, delete the original and rename the temporary to the original. This should seem to be the original file when complete. Windows performs tunneling on both FAT and NTFS file systems to ensure long/short file names are retained when 16-bit applications perform this safe save operation.

How to See the Effects of Tunneling

1. Create a file called file1.
2. Wait for one to two minutes.
3. Create a file called file2.
4. Perform a DIR /TC. Please note the creation times.
5. Rename file1 to file.
6. Rename file2 to file1.
7. Perform a DIR /TC. Please note the creation times are identical.

Both files now show the same creation times; they are identical to the original FILE1 creation time and is expected behavior with tunneling enabled.

Tunneling cache time can be adjusted from the default time of 15 seconds, or if tunneling capabilities are undesirable, it can be disabled by adding a value in the Windows Registry.

If tunneling is disabled, applications that use this safe save method can lose the name they are unaware of, usually the LFN, and the rediscovery of shortcut targets could be impaired since the creation timestamps cannot remain constant for files manipulated by such apps. Creation timestamp maintenance is possible in the absence of tunneling if an application is smart enough. The same is not true for the long filenames.

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

How to Increase Tunneling Cache Time

Follow these steps, and then quit Registry Editor:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following key in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type MaximumTunnelEntryAgeInSeconds, and then press ENTER.
5. On the Edit menu, click Modify.
6. Type 30, and then click OK.

How to Disable Tunneling

Follow these steps, and then quit Registry Editor:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following key in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem
3. On the Edit menu, point to New, and then click DWORD Value.
4. Type MaximumTunnelEntries, and then press ENTER.
5. On the Edit menu, click Modify.
6. Type 0, and then click OK.