Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Per Jessen]

Carlos E. R. wrote:

> On 2016-09-05 08:47, Per Jessen wrote:
>> Carlos E. R. wrote:
> 
> 
>> I'm beginning to wonder
>> 
>> - if Dom0 is actually a little of out sync, ntp would slowly be
>> adjusting the clock, and I guess systemd would notice that.
> 
> I doubt that systemd would notice unless carefully watching for it,

I agree, so it clearly does.  I suspect there is some event that is
triggered when time make a jump or time is not monotone. 



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Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Carlos E. R.]

On 2016-09-05 08:47, Per Jessen wrote:
> Carlos E. R. wrote:


> I'm beginning to wonder 
> 
> - if Dom0 is actually a little of out sync, ntp would slowly be
> adjusting the clock, and I guess systemd would notice that. 

I doubt that systemd would notice unless carefully watching for it,
because normally ntpd adjusts the speed of the clock, not the value of
the clock itself. Programs can not notice that unless they compare the
clock with some other reference and see the difference.

Programs can only notice if the value of the clock change. If it jumps
suddenly skipping intermediate values.

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Cheers / Saludos,

Carlos E. R.
(from 13.1 x86_64 "Bottle" at Telcontar)



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Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Per Jessen]

Carlos E. R. wrote:

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> 
> On 2016-09-04 18:39, Per Jessen wrote:
> 
>> I don't know if anybody cares much, but on a xen guest running
>> Leap422b1 with ntp disabled, I no longer see those "Time has been
>> changed" messages apart from once or twice.  (I used to have them
>> twice a minute).  I'll leave it running for a while and see what
>> happens.
>> 
>> Does anyone here have an opinion on the matter?
> 
> That's what I see on my vmware player guest, too.
> 
> I'd guess that systemd has its own clock adjustment service running
> and it doesn't like ntp touching the clock as well. What that service
> might be, I don't know.

I doubt that very much. Besides, why should openSUSE have ntp running by
default then?

I'm beginning to wonder 

- if Dom0 is actually a little of out sync, ntp would slowly be
adjusting the clock, and I guess systemd would notice that.  

- if Dom0 is actually sync'ed, no adjustment is needed, nothing for
systemd to notice. 


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Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Carlos E. R.]

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On 2016-09-04 18:39, Per Jessen wrote:

> I don't know if anybody cares much, but on a xen guest running 
> Leap422b1 with ntp disabled, I no longer see those "Time has been 
> changed" messages apart from once or twice.  (I used to have them
> twice a minute).  I'll leave it running for a while and see what
> happens.
> 
> Does anyone here have an opinion on the matter?

That's what I see on my vmware player guest, too.

I'd guess that systemd has its own clock adjustment service running
and it doesn't like ntp touching the clock as well. What that service
might be, I don't know.

- -- 
Cheers / Saludos,

Carlos E. R.

  (from 13.1 x86_64 "Bottle" (Minas Tirith))
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Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Per Jessen]

Per Jessen wrote:

> Carlos E. R. wrote:
> 
>> On 2016-09-02 16:00, Per Jessen wrote:
>>> Continued from opensuse-factory:
>> 
>> (by the way, this was
>> https://bugzilla.novell.com/show_bug.cgi?id=773323)
> 
> I didn't know that one, but it seems to be about local vs UTC time?
> 
 But if you run the ntp daemon on guests, it tries to adjust the
 speed of the clock there. That's how it works.
>>> 
>>> Of course, that's what you want it to do.  You want the guest clock
>>> synchronized.  It would appear to be a good idea to have ntp on the
>>> xen Dom0 govern the clock for all the guests, but in my experience
>>> systemd does not like that.
>> 
>> Yes, we want the clock synchronized, but not by adjusting the speed
>> on guests. Just copied from the host. It adds load on the virtualized
>> hardware.
> 
> Yes, the obvious way seems to be to just leave the guests to trust the
> synchronized clock from the Dom0.  The load is negligible, especially
> on a typical virtual host.

I don't know if anybody cares much, but on a xen guest running 
Leap422b1 with ntp disabled, I no longer see those "Time has been
changed" messages apart from once or twice.  (I used to have them twice
a minute).  I'll leave it running for a while and see what happens. 

Does anyone here have an opinion on the matter?


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Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Per Jessen]

Carlos E. R. wrote:

> On 2016-09-02 16:00, Per Jessen wrote:
>> Continued from opensuse-factory:
> 
> (by the way, this was
> https://bugzilla.novell.com/show_bug.cgi?id=773323)

I didn't know that one, but it seems to be about local vs UTC time? 

>>> But if you run the ntp daemon on guests, it tries to adjust the
>>> speed of the clock there. That's how it works.
>> 
>> Of course, that's what you want it to do.  You want the guest clock
>> synchronized.  It would appear to be a good idea to have ntp on the
>> xen Dom0 govern the clock for all the guests, but in my experience
>> systemd does not like that.
> 
> Yes, we want the clock synchronized, but not by adjusting the speed on
> guests. Just copied from the host. It adds load on the virtualized
> hardware.

Yes, the obvious way seems to be to just leave the guests to trust the
synchronized clock from the Dom0.  The load is negligible, especially
on a typical virtual host.  



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Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Carlos E. R.]

On 2016-09-02 16:48, Carlos E. R. wrote:
> On 2016-09-02 16:00, Per Jessen wrote:
>> Continued from opensuse-factory:
> 
> (by the way, this was
> https://bugzilla.novell.com/show_bug.cgi?id=773323)
> 
> 
>>> But if you run the ntp daemon on guests, it tries to adjust the speed
>>> of the clock there. That's how it works.
>>
>> Of course, that's what you want it to do.  You want the guest clock
>> synchronized.  It would appear to be a good idea to have ntp on the xen
>> Dom0 govern the clock for all the guests, but in my experience systemd
>> does not like that. 
> 
> Yes, we want the clock synchronized, but not by adjusting the speed on
> guests. Just copied from the host. It adds load on the virtualized hardware.
> 
> I know that vmware does recommends not running ntpd in guests, but
> instead use:
> 
> tools.syncTime = "TRUE"
> 
> I'll try again to find a source for this.

I have not found the paper where I go that info; instead got another that 
differs.

One old document was  but that link no longer 
works. 
I read a note on one of the posts to google for vmware_timekeeping.pdf, which I 
found here:



This one has recommendations to use vmware internal clock adjusting, or ntp, 
with appropriate configs. And be careful to never use both.

The interesting section is "Synchronizing Virtual Machines and Hosts with Real 
Time" in page 14.
Besides that, the document is very interesting read about PC clocks in general.


+++-
VMware Tools periodic clock synchronization has the advantage that it is aware 
of the virtual machine’s 
built‐in catch‐up and interacts properly with it. If the guest operating system 
clock is behind real time by more 
than the known backlog that is in the process of being caught up, VMware Tools 
resets the clock and informs 
the virtual machine to stop catching up, which sets the backlog to zero. An 
additional advantage of VMware 
Tools clock synchronization is that it does not require networking to be set up 
in the guest. However, at this 
writing, VMware Tools clock synchronization has a serious limitation: it cannot 
correct the guest clock if it gets 
ahead of real time (except in the case of NetWare guest operating systems). 
This limitation applies only to 
periodic clock synchronization. VMware Tools does a one‐shot correction of the 
virtual machine clock that 
may set it either backward or forward in two cases: when the VMware Tools 
daemon starts (normally while 
the guest operating system is booting), and when a user toggles the periodic 
clock synchronization feature 
from off to on.

Native synchronization software has the advantage that it is generally prepared 
to deal with the virtual 
machine clock being either ahead of or behind real time. It has the 
disadvantage that it is not aware of the 
virtual machine’s built‐in catch‐up and thus typically does not synchronize 
time as well in a virtual machine 
as it does when run directly on physical hardware. 

One specific problem occurs if native synchronization software happens to set 
the guest operating system 
clock forward to the correct time while the virtual machine has an interrupt 
backlog that it is in the process of 
catching up. Setting the guest operating system clock ahead is a purely 
software event that the virtual machine 
cannot be aware of, so it does not know that it should stop the catch‐up 
process. As a result, the guest operating 
system clock continues to run fast until catch‐up is complete, and it ends up 
ahead of the correct time. 
Fortunately, such events are infrequent, and the native synchronization 
software generally detects and corrects 
the error the next time it runs.

Another specific problem is that native synchronization software may employ 
control algorithms that are 
tuned for the typical rate variation of physical hardware timer devices. 
Virtual timer devices have a more 
widely variable rate, which can make it difficult for the synchronization 
software to lock on to the proper 
correction factor to make the guest operating system clock run at precisely the 
rate of real time. As a result, the 
guest operating system clock tends to oscillate around the correct time to some 
degree. The native software 
may even determine that the timer device is broken and give up on correcting 
the clock. 

Despite these potential problems, however, testing has shown that NTP in 
particular behaves fairly well in a 
virtual machine when appropriately configured (see “Using NTP in Linux and 
Other Guests” on page 17). 
NTP is prepared for some of its readings to be anomalous because of network 
delays, scheduling delays on the 
local host, and other factors and is effective at filtering out such readings.
Generally, it is best to use only one clock synchronization service at a time 
in a given virtual machine to ensure 
that multiple services do not attempt to make conflicting changes to the clock. 
So if you are using native 
synchro

Re: [opensuse-virtual] should ntp be governing the clock in a guest system?

[Carlos E. R.]

On 2016-09-02 16:00, Per Jessen wrote:
> Continued from opensuse-factory:

(by the way, this was
https://bugzilla.novell.com/show_bug.cgi?id=773323)


>> But if you run the ntp daemon on guests, it tries to adjust the speed
>> of the clock there. That's how it works.
> 
> Of course, that's what you want it to do.  You want the guest clock
> synchronized.  It would appear to be a good idea to have ntp on the xen
> Dom0 govern the clock for all the guests, but in my experience systemd
> does not like that. 

Yes, we want the clock synchronized, but not by adjusting the speed on
guests. Just copied from the host. It adds load on the virtualized hardware.

I know that vmware does recommends not running ntpd in guests, but
instead use:

tools.syncTime = "TRUE"

I'll try again to find a source for this.


> I looked into in March 2015 when I was virtualising some systems, and
> found the Novell page above. 
> 
> The Xen FAQ:  https://wiki.xen.org/wiki/Xen_FAQ_DomU also recommends
> running ntp in each DomU. 
> 
> Finally, Werner Flamme wrote:
> 
>> When you install SAP systems on top of Xenified VMs, there is the
>> recommendation to do the latter: disable the sync and run a separate
>> ntpd in the DomU. This is also a recommendation for VMs inside a
>> VMWare environment - the problem ist the same there.
> 
> Back in March last year I did ask what the typical practice was, but
> noone had any suggestions.

This is against what I read the vmware people say.

-- 
Cheers / Saludos,

Carlos E. R.
(from 13.1 x86_64 "Bottle" at Telcontar)



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