I recently encountered a very annoying problem on my Windows 10/Server 2016 VMs running on XenServer 7.1 CU1.
On startup of the VMs the time was advanced 54 minutes when XenInterface/XenAgent started, when the w32time service started on the VM the time was then synchronized to the DCs of the domain and was ok again.
My issues I got was that some of my VMs lost connection to the domain - because the time was off with more than 5 minutes, and this was happening randomly - depending on when the Xen Agents was starting. The impact for me was that the startup scripts configured in the domain was not running on the VMs...
My troubleshooting was to check NTP settings on the hosts, on the DCs and the HW clock on the Hosts - all to no avail - meaning no clock issues on any of them. Also the timeoffset on the VM metadata I tried to set differently - but also did not change the behaviour.
So in the end, my only workaround left was to try to disable the timesync between the VMs XenAgent (XenInterface) and the host. But no official documentation stated that this was possible..
However, after a bit of digging I found in the Xen source code for the XenAgent - it seemed there was a registry setting inside the VM that was checked for a specific value - if that was found it would synchronize the VMs' time with the host. So in order to disable that you can just set tha t value to anything else - or remove the value all together would also do the trick I believe.
The registry key/value to disable time sync with host (works on Windows 10 VMs (tested on 1709), Server 2016, XenServer tools 7.1.1305, XenServer 7.1 CU1):
Question
Patrik Nyberg
I recently encountered a very annoying problem on my Windows 10/Server 2016 VMs running on XenServer 7.1 CU1.
On startup of the VMs the time was advanced 54 minutes when XenInterface/XenAgent started, when the w32time service started on the VM the time was then synchronized to the DCs of the domain and was ok again.
My issues I got was that some of my VMs lost connection to the domain - because the time was off with more than 5 minutes, and this was happening randomly - depending on when the Xen Agents was starting. The impact for me was that the startup scripts configured in the domain was not running on the VMs...
My troubleshooting was to check NTP settings on the hosts, on the DCs and the HW clock on the Hosts - all to no avail - meaning no clock issues on any of them. Also the timeoffset on the VM metadata I tried to set differently - but also did not change the behaviour.
So in the end, my only workaround left was to try to disable the timesync between the VMs XenAgent (XenInterface) and the host. But no official documentation stated that this was possible..
However, after a bit of digging I found in the Xen source code for the XenAgent - it seemed there was a registry setting inside the VM that was checked for a specific value - if that was found it would synchronize the VMs' time with the host. So in order to disable that you can just set tha t value to anything else - or remove the value all together would also do the trick I believe.
The registry key/value to disable time sync with host (works on Windows 10 VMs (tested on 1709), Server 2016, XenServer tools 7.1.1305, XenServer 7.1 CU1):
HKLM\Software\Citrix\XenTools\HostTime="Local"
HKLM\Software\Wow6432Node\Citrix\XenTools\HostTime="Local"
Default value is HostTime="UTC" - so set anything else than that - should disable sync with the host.
For me the problem went away completely and I have implemented this registry setting on all my VMs now.
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