Re: why the clock become very slow?
- From: Le Wen <wenle@xxxxxxxxxx>
- Date: Fri, 27 Nov 2009 09:30:53 +0800
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@xxxxxxxxx>
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2009-11-27 02:12
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Subject
Re: why the clock become very slow?
I think that's normal then.http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006427
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:
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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