Re: Question on kvm_clock working ...

[Ronen Hod <rhod@redhat.com>]

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On 09/09/2011 06:28 PM, al pat wrote:
>
> We are doing an experiment with kvm-clock to validate its 
> effectiveness, particularly when running NTP on the host to make sure 
> the host’s clock stays properly sync.
> Our observations leads us to a few unanswered questions, including the 
> possibility of a bug (our our misunderstanding of how kvm_clock should 
> work).
>
> Our understanding is that kvm_clock will help sync the clock between 
> the host and the guest. We do not observe this to happen in reality 
> and thus this question.
>
> We are using Ubuntu 11.04 on the host and the guest.
>
> The command we issue to launch the VM is the following:
>
> $ sudo kvm -m 500 -rtc clock=host guestos.img
>
> We also arranged for Ubuntu to show the seconds on the clock displayed 
> in the menu.
>
> Observation 1:
> Upon launching the VM, we see a time difference between the 2 clock 
> ranging from 1 to 2 seconds.
>
> Observation 2:
> If we change the date on the host (with a command such as “date --set 
> 10:00:00 AM Sep 9, 2011”), the time on the guest remains the same, 
> unaffected.
>
> Observation 3:
> After running for a while without NTP on the host, we run “ntpdate” to 
> sync up the host, but the guest stick with whatever previous time.

You probably meant "ntpd -q"

>
>
> Another test we will run is to have ntpd on the host and wait for an 
> extended time to see if the guest drifts away from that original 1 or 
>  2 second lag. In the meantime, we are asking you for some input in 
> this regards:
> Questions
> -What does the “–rtc clock” option is supposed to mean exactly? 
>  According to the man page, the guest should get its time from the 
> host, but neither date nor an “ntpdate” affected the clock on the guest.
> -What are the other options that we should use?
>
>        -rtc [base=utc|localtime|date][,clock=host|vm][,driftfix=none|slew]
>           Specify base as "utc" or "localtime" to let the RTC start at the
>           current UTC or local time, respectively. "localtime" is required
>           for correct date in MS-DOS or Windows. To start at a 
> specific point
>           in time, provide date in the format "2006-06-17T16:01:21" or
>            "2006-06-17". The default base is UTC.
>
>           By default the RTC is driven by the host system time. This 
> allows
>           to use the RTC as accurate reference clock inside the guest,
>           specifically if the host time is smoothly following an accurate
>           external reference clock, e.g. via NTP.  If you want to 
> isolate the
>           guest time from the host, even prevent it from progressing 
> during
>           suspension, you can set clock to "vm" instead.
>
>           Enable driftfix (i386 targets only) if you experience time drift
>           problems, specifically with Windows' ACPI HAL. This option 
> will try
>           to figure out how many timer interrupts were not processed 
> by the
>           Windows guest and will re-inject them.
>
>
> Can someone shed light on what we are missing? Any pointers will be 
> helpful.
>
> Thanks
> -a
>


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