Slow performance when you try to access resources on your Virtual Server 2005 host computer from a guest virtual machine (888750)

Important This article contains information about how to modify the registry. Make sure to back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows registry

SYMPTOMS

When you try to access resources on your Microsoft Virtual Server 2005 host computer from a guest virtual machine, the performance is very slow. When you try to copy a large file, the file copy may take minutes when you expect it to take seconds. Or, the large file transfer may not be completed. You may also experience that the guest virtual machine or other guest virtual machines that are attached to the same virtual network may disconnect from the virtual network.

This problem may also cause actual non-virtual network connections to the host machine to be disconnected. You may see an event log entry that is similar to the following event log entry :Event ID: 2025
Source: SRV
Description: The server has detected an attempted Denial-of-Service attack from client \\computername, and has disconnected the connection.

CAUSE

This problem may occur if you have Transmission Control Protocol (TCP) segmentation offloading enabled on the host computer's network adapter. The Digital Equipment Corporation (DEC)/Intel 21140A chipset that Virtual Server 2005 emulates does not support TCP segmentation offloading. Therefore, the Virtual Server Network Driver Interface Specification (NDIS) filter driver must perform the segmentation. This use of the NDIS filter driver may also contribute to slow performance.

Note This problem is most likely to occur when you use a gigabit network adapter that has the virtual network attached to the adapter.

WORKAROUND

To work around this problem, use one of the following methods.

Method 1

Enable local area network (LAN) routing on the host computer, and then move the virtual network to the Microsoft Loopback Adapter.

Note To perform this procedure, your host computer must be running Microsoft Windows Server 2003.

To enable LAN routing on the host computer, follow these steps:

Click Start, point to Administrative Tools, and then click Routing and Remote Access.
Right-click your server name.
If remote access is already enabled, follow these steps:
1. Click Properties, and then click the General tab.
2. Make sure that the Router check box is selected, and then click OK.
If remote access is disabled, follow these steps:
1. Click Configure and Enable Routing and Remote Access.
  
  The Routing and Remote Access Server Setup Wizard starts.
2. Click Next.
3. On the Configuration page, click Custom configuration, and then click Next.
4. On the Custom Configuration page, click to select the LAN routing check box.
5. Click Next, and then click Finish.
6. Click Yes if you are prompted to start the Routing and Remote Access service.

For information about how to move the virtual network to the Microsoft Loopback Adapter, see the "Using Microsoft Loopback Adapter" and the "Setting up virtual networks" topics in Virtual Server 2005 Help.

Method 2

Disable TCP task offloading in the registry of the host computer. To do this, follow these steps.

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

Click Start, click Run, type regedit, and then click OK.
Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
Click Edit, point to New, and then click DWORD Value.
Type DisableTaskOffload as the entry name, and then press ENTER.
Right-click DisableTaskOffload, and then click Modify.
In the Value data box, type a value of 1, and then click OK.
Quit Registry Editor.
Restart your computer.

MORE INFORMATION

If you want to isolate the problem even more, do not add the DisableTaskOffload registry entry that is described in the "Resolution" section. Instead, disable the task offload properties on the Advanced tab of the network adapter's Properties dialog box one at a time until you discover which one is causing the performance issue. To do this, follow these steps.

Warning When you perform this procedure, guest virtual machines that are attached to the same virtual network may temporarily disconnect from the virtual network.

Click Start, click Run, type ncpa.cpl, and then click OK.
Right-click your network adapter, and then click Properties.
Click the General tab, and then click Configure.
Click the Advanced tab.
In the Property box, click any one of the following properties.

Note We recommend that you start with the Offload TCP Segmentation property. The problem is probably caused by the Offload TCP Segmentation property because full support for this feature is not implemented in the Vpcnets2.sys driver.
- Offload TCP Segmentation
- Offload Receive IP Checksum
- Offload Receive TCP Checksum
- Offload Transmit IP Checksum
- Offload Transmit TCP Checksum
In the Value list, click Off, and then click OK.
Test to see if the performance issue has been resolved. If the problem still occurs, re-enable the property that you turned off, and then disable a different task offload property. Test if performance has improved after you disable each property.