why the clock become very slow?

[Le Wen]

Hi,

I don't think so, 

as you can read from page 15,

"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."

"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
synchronization software, we suggest turning VMware Tools periodic clock 
synchronization off."

You can use any way to synchronize your vm, I used to adjust the kernel 
parameters, it works well in my ORACLE RAC.

Rds,

Winner




Eugene Vilensky <evilensky at gmail.com> 
Sent by: redhat-list-bounces at redhat.com
2009-11-27 02:12
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Subject
Re: why the clock become very slow?







> I think that's normal then.
>
> You can do the following according to your linux version:
>
> 1.Synchronize your linux time using vmware-tools
>
> 2.Adjust your kernel parameters base on the following:
>
> 
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006427


I'm surprised that after all those kernel parameters, the NTP
configuration section of KB 1006427 says this:

"NTP Recommendations
Note: In all cases use NTP instead of VMware Tools periodic time
synchronization. Also, you may need to open the firewall (UDP 123) to
allow NTP traffic."

Is this a flat-out recommendation to use NTP anywhere and everywhere
possible?  Because there is no such explicit recommendation in this:
http://www.vmware.com/pdf/vmware_timekeeping.pdf

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