Over the past few months IC Solutions has gathered information from various sources on how to avoid data corruption in shared files. The solution applies to any application that runs on Windows 95 and shares files on an NT 4 server. These problems manifest themselves in CIC as table corruptions, index out of date errors and general data file corruption problems. It is caused by the local cache satisfying read requests from, and holding onto data written to, a shared remote data file.

The bug is in the VREDIR.VXD system file shipped with Windows 95 (even with service pack 1) and OSR2. Microsoft has released a new version of VREDIR.VXD (4.00.1116) and VNETSUP.VXD (4.00.1112) that eliminate the problems with the local cache. You can download the new versions from http://support.microsoft.com/download/support/mslfiles/vrdrupd.exe. The file is 215,728 bytes in length. When run, it starts up a standard setup to install the new VREDIR.VXD (156,773 bytes, 11-Sep-97 11:16am) and the new VNETSUP.VXD (17,595 bytes, 30-May-97 11:12am). You'll have to reboot your Windows 95 system to complete the installation.

The next step is to add a new registry entry to the Windows 95 system using REGEDIT.EXE:

Key: HKEY_LOCAL_MACHINE/ System/CurrentControlSet/ Services/VxD/VREDIR
Value: DiscardCacheOnOpen (binary value)
Data: 01

Once you've exited REGEDIT, you will need to reboot your machine for the new setting to take effect. Only the new VREDIR.VXD will understand this setting, previous ones will ignore it.

If you are using a Windows NT 4 Server as your file manager (in other words, the shared data files reside on a Windows NT 4 Server machine), there's one more step to take: You need to turn off optimistic locking at the server. You need to add a new registry key on the NT 4 Server (or modify the current one if it exists).

Key: HKEY_LOCAL_MACHINE/ System/CurrentControlSet/ Services/LanmanServer/ Parameters
Value: EnableOpLocks (DWORD value)
Data: 0

These changes will have to be made to every Windows 95 and NT 4 Server PC on the network. If your application gets installed on a new network then it is up to you to ensure that all workstations and servers on the new network are also upgraded. Note that using Registry Editor incorrectly can cause serious, system-wide problems that may require you to reinstall your operating system to correct them.

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