Questions about KVM-Clock and NTP for Ubuntu Guests

Mon Apr 22 21:56:43 UTC 2013

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This has been updated here:
https://help.ubuntu.com/community/KVM/FAQ#Should_ntp_be_used_for_time_synchronisation.3F
- --chris


On 04/23/2013 02:26 PM, Chris J Arges wrote:
> Hi everybody, I have a fairly in-depth set of questions about 
> kvm-clock and synchronization. I've tried to pare these down in to
> a set of questions that may help clarify concerns of users that
> utilize Ubuntu as a kvm guest. I've also done some (rough)
> experiments to answer some of the basic questions.
> 
> My goal is for these to clarify [0] and be eventually update the
> wiki.
> 
> = Questions =
> 
> * Why precisely does Ubuntu recommend _not_ to use NTP in KVM
> guests [0]? Some distros and people [9][2][5] recommend using NTP
> in their guests, as well as the KVM FAQ [1]. If kvm-clock
> accomplishes proper clock synchronization, is NTP just a waste of
> time, or could it actually cause bugs and additional problems?
> Would NTP fail because of system time drift or other issues [5]?
> 
> * Does kvm-clock propagate the host system time to guests? Yes, it
> exposes the host system time as shown in the below experiments.
> Does this also expose UTC corrections such as leap seconds [5]?
> 
> * How can Guest VM's update their system time from hwclock? 
> Executing "hwclock --hctosys" updates the system time, but perhaps 
> there is a better way?
> 
> * What happens when a system suspends and resumes? It looks like
> the system still maintains the proper timing.
> 
> = Experiment =
> 
> * Assuming you have a clean KVM guest setup. For my experiment I
> ran a 3.8.0-19 kernel in an raring amd64 server install.
> 
> * For each experiment record the following in the guest: dmesg |
> grep "setting system clock" echo "date +%s output = " $(date +%s) 
> hwclock --debug | grep "Hw clock" clockdiff 192.168.122.1 #or
> whatever the host machine IP is
> 
> * Compare to time when you start the vm and whenever you run the
> script.
> 
> 1) Boot guest and record output, it should be somewhat close in
> time. * Host before boot: 1366742263 [    0.528148] rtc_cmos 00:00:
> setting system clock to 2013-04-23 18:37:48 UTC (1366742268)
> 
> * Host when executing script: 1366742312 date +%s output =
> 1366742313 Hw clock time : 2013/04/23 18:38:33 = 1366742313 seconds
> since 1969 host=192.168.122.1 rtt=750(187)ms/0ms delta=-40ms/-40ms
> Tue Apr 23 13:38:33 2013
> 
> So this matches expectations.
> 
> 2) Change host system time and record output afterwards. * Host
> when executing script: 924374443 date +%s output = 1366742454 Hw
> clock time : 1999/04/17 18:40:44 = 924374444 seconds since 1969 
> host=192.168.122.1 rtt=750(187)ms/0ms delta=-10513ms/-10513ms Tue 
> Apr 23 13:40:54 2013
> 
> Looks like hwclock is correct, but 'date' doesn't reflect the 
> changed time. Running 'sudo hwclock --hctosys' fixes this: $ sudo
> hwclock --hctosys ubuntu at ubuntu:~$ echo "date output = " $(date
> +%s) date output =  924374611
> 
> 2) Set host system time to something bogus, boot and check output. 
> * Host before boot: 86415 [    0.581315] rtc_cmos 00:00: setting
> system clock to 1970-01-02 00:00:20 UTC (86420)
> 
> * Host when executing script: date +%s output = 1366742803 Hw clock
> time : 1970/01/02 00:01:03 = 86463 seconds since 1969 
> host=192.168.122.1 rtt=562(280)ms/0ms delta=18859219ms/18859219ms 
> Tue Apr 23 13:46:45 2013
> 
> So again looks like hwclock is correct, but system time isn't 
> updated from hwclock on boot.
> 
> 4) Boot machine and record data. Then suspend machine for X
> minutes, resume and record data afterwards. Paused the VM for about
> 4m. ubuntu at ubuntu:~$ date +%s 1366742933 ubuntu at ubuntu:~$ date +%s 
> 1366743185 This delta matches the pause of 4m, so the clock looks
> proper.
> 
> Below is a list of various references I looked at to help answer
> these questions. Thanks, --chris j arges
> 
> [0] 
> https://help.ubuntu.com/community/KVM/FAQ#Should_ntp_be_used_for_time_synchronisation.3F
>
> 
[1]
> http://www.linux-kvm.org/page/FAQ#I.27m_experiencing_timer_drift_issues_in_my_VM_guests.2C_what_to_do.3F
>
> 
[2] http://s19n.net/articles/2011/kvm_clock.html
> [3] 
> http://rogierm.redbee.nl/blog/2011/11/07/time-synchronisation-in-kvm-guest-to-ntp-or-not-to-ntp/
>
> 
[4] https://rwmj.wordpress.com/tag/kvmclock/
> [5] 
> http://thread.gmane.org/gmane.comp.emulators.kvm.devel/96835/focus=96981
>
> 
[6]
> http://www.mjmwired.net/kernel/Documentation/virtual/kvm/timekeeping.txt
>
> 
[7] https://lkml.org/lkml/2010/4/15/353
> [8] http://tldp.org/HOWTO/Clock-2.html [9] 
> https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Host_Configuration_and_Guest_Installation_Guide/chap-Virtualization_Host_Configuration_and_Guest_Installation_Guide-KVM_guest_timing_management.html
>
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