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xen-users
Re: [Xen-users] Time is off by an hour in my XEN vm
Thanks for your detailed explanation, but the problem is the server
itself runs Linux and the clock is set to UTC. Other Xen VM's don't
have any problems and use the same settings as my VM.
Frank
Op 13 mrt 2008, om 23:15 heeft Ralf Kaestner het volgende geschreven:
"Frank Groeneveld" wrote:
Yes, that will probably work, but it used to work without such
tricks.
I'm sure it's just a setting in my VM or something like that.
I think to know what that issue might be, and it is most likely not
a Xen Software problem at all, but a general virtualization issue. I
had to deal with similar problems in the past as well and collected
the following information from google + different other sources.
As you might know, there is a posix standard that defines any
computer's RTC (CMOS Real Time Clock) to be UTC (or GMT or however
you call it). Anyhow, as you probably also might or might not
assume, Microsoft does not follow that standard and interprets the
RTC as localtime (GMT +/- X). It shows +1h on your system, so I
assume you are GMT+1 (or UTC+1). There are probably reasons for
Microsoft doing so, even if we can't really spot them - most likely
because of backward compatibility -who knows ;) As you can see -
there is a discrepancy between your host, running linux, following
posix, and you guest, that gets no own RTC from xen based on the
nature of xen virtualization and therefore needs to share the real
RTC with linux but the OS is not following posix.
There is a RealTimeIsUniversal registry flag hidden in the windows
registry that can be set (its not in by default) to let Windows
interpret the RTC as UTC as well.
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control
\TimeZoneInformation] "RealTimeIsUniversal"=dword:00000001
This possibility seems to be there as a leftover from the days when
NT still ran on RISC machines with UTC RTCs but is not even official
documented anywhere as far as I've seen. So I would not assume that
it will be there forever. The RTIU registry key is still active
within the NT kernel for at least XP and 2003 and works in a way to
tell Windows to interpret the Hardware clock as UTC, BUT - only on
boot time (actually the kernel seems to read that key directly,
check the NT kernel with "strings"). So you can use that as a
workaround whenever you run your linux host posix and windows on top.
I have recognized that windows has another special "gotcha" you need
to know regarding the clock that is not well documented. Windows
tries to sync time exactly every hour (starting 1h after boot) with
the hardware CMOS clock AS LONG AS you either don't have NTP
configured or none of the configured NTP server(s) is reachable. In
that case, the RealTimeIsUniversal key is not interpreted by the
running OS - the reg key seems to be only used by the kernel itsef
at boot time - you can find the string within the kernel binary -
but nowhere else in the OS. So this may become another issue, even
if you set the windows registry key above. As long as NTP sync is
configured and any NTP server is reachable on bootime+1h+1h+...,
Windows does not sync with CMOS anymore but uses the NTP as time
source. As soon as I have a working NTP sync in place, the clock
does not get set back to UTC anymore, otherwise it would even get
set back to localtime every hour..
Another option would be to set your bios to localtime and tell linux
to ignore the posix standard - linux deals much better with that
than windows with the counterpart. So you would have to tell the
linux xen host to interpret the RTC as local (check http://man.he.net/man8/clock)
. Most people recommend that when one runs windows and linux on the
same host (e.g. within xen environments) as the best practice.
hope this helped
cheers
Ralf
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