How to Disable Receive-Side Copy (RSC) on XenServer 6.x

Summary

This article describes how to disable the Receive-Side Copy on XenServer 6.x

Requirements

Access to Windows Virtual Machine and Registry Editor
Access to XenServer command line interface to monitor logs

Background

In XenServer 6.x, a new feature called Receive-Side Copy (RSC) is enabled by default in Windows virtual machines with XenTools installed.

RSC moves the processing required for network traffic from Domain 0 (netback process) into the virtual machine (netfront process) thereby reducing the load on the control domain when you run large numbers of virtual machines.

In some scenarios, you might have to disable this feature to improve the virtual machine performance.

Known Issue

With RSC enabled, a virtual machine virtual interface cannot count transmitted packets. For example, the following is an output for the ifconfig command for a Windows virtual machine that has transmitted a large amount of network traffic:

Windows 7 (64-bit)- vif4.0
RX packets:991371 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:32
RX bytes:41558972 (39.6 MiB) TX bytes:0 (0.0 b)

Note: This issue is addressed in Hotfix XS602E005 for XenServer 6.0.2 (see item #7)

Procedure

This procedure contains a couple of options you can perform. Depending on your requirements, you can perform option 1, option 2, or both the options.

Caution! Using Registry Editor incorrectly can cause serious problems that might require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk. Be sure to back up the registry before you edit it.

Option 1

To disable the Receive-Side Copy inside the Windows Virtual Machine, complete the following procedure:

Apply the following registry key to the Windows virtual machine:
Note: You can copy the text and save it as a .reg file and then import the file into the registry.
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\xenvif\Parameters]
"ReceiverMaximumProtocol"=dword:00000000
Run the following command on the XenServer host, to monitor the ReceiverMaximumProtocol value. When the virtual machine restarts, the value must be 0.
tail -f /var/log/messages | grep ReceiverMaximumProtocol
Restart the virtual machine to apply the registry change and monitor the messages log to verify if the RSC is disabled.
To re-enable RSC, you must delete the ReceiverMaximumProtocol registry key and restart the virtual machine.

Option 2

To disable RSC in Dom0 for all virtual machines on the host, run the following command at the CLI:
/opt/xensource/libexec/xen-cmdline --set-dom0 xen_netback.netback_max_rx_protocol=0

Refer CTX136830 - How to Disable Receive-Side Copy (RSC) in Dom0 for more information on disabling RSC in Dom0.

More Information

CTX136830 - How to Disable Receive-Side Copy (RSC) in Dom0
RSC and the Impact on XenServer Scalability