How to enable experimental write-behind caching for redirected drives in Parallels RAS

When a file is saved to a redirected (mapped) drive from a Parallels RAS session, the write can take noticeably longer than saving the same file locally — sometimes tens of seconds, depending on the file size and network conditions. Starting with Parallels RAS 21.2, an experimental write-behind caching option is available for the redirected drive driver. When enabled, file writes are cached locally on the RD Session Host and flushed to the redirected drive in the background, which can significantly reduce the delay the user experiences when saving.

This feature is disabled by default and is currently enabled using a registry key on the RD Session Host. There is no RAS Console option for it yet — UI-based configuration is planned for a future release.

Note: This is an experimental feature. Test it in a non-production environment before enabling it on production RD Session Hosts.

How to enable write-behind caching

1. Sign in to the RD Session Host where the redirected drive driver (RasRdpFs) is installed — this is the RAS session host that users connect to, not the client machine.
2. Open Registry Editor (regedit.exe) with administrative privileges.
3. Navigate to:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RasRdpFs

4. Create a new DWORD (32-bit) Value named WriteCacheEnabled (if it does not already exist).
5. Set its value to 1 to enable write-behind caching.
6. Restart the RasRdpFs service for the change to take effect. Open an elevated Command Prompt on the RD Session Host and run:

   net stop RasRdpFs
   net start RasRdpFs

   A full host restart is not required.

7. Reconnect with Parallels Client and test saving a file to a redirected drive.

Example: Registry file

The following .reg file content can be imported on the RD Session Host to set the value automatically. Save it as a .reg file and double-click it, then restart the driver or the host as described above.

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RasRdpFs]
"WriteCacheEnabled"=dword:00000001

How to disable write-behind caching

To revert to the default behavior, set WriteCacheEnabled to 0, or delete the value entirely, and restart the RasRdpFs service again:

net stop RasRdpFs
net start RasRdpFs

What to expect

Note: Because the cache is write-behind, there is a short delay between an application reporting that a save completed and the data being fully written to the redirected drive. Keep this in mind when evaluating the feature, particularly for workflows where another process reads the file immediately after it is saved, or where the session might be disconnected unexpectedly.