why doesn't VMWare Tools' time synchronization work when clock runs fast?

[mykmelez]

My Ubuntu Linux 7.04 VM's clock has been running fast lately. I'm not sure why. Perhaps it started after I configured the VM to use two virtual processors.

In any case, the time synchronization feature in VMWare Tools only synchronizes clocks that run slow, not those that run fast, per file:///usr/lib/vmware-tools/hlp/tools_options.htm , which says:

Time synchronization between the virtual machine and the host operating system – When

enabled, this option periodically (every second) checks whether the guest operating

system time is lagging behind the host operating system time. If it is, the guest operating

system time is bumped to match the host operating system time.

You should enable this option if you notice that time does not elapse inside your virtual

machine as fast as it does in your host operating system.

You can synchronize the time between the guest and host operating systems only when

you set the clock in the guest operating system to a time earlier than the time set in the

host.

Why is that? Isn't it useful to synchronize clocks that run fast? It certainly would be in my case. I keep thinking it's later than it is (I spend most of my work time in that VM).

[vmware34]

Hey,

Did you ever get an answer on this. I did not see an update and I have been doing some testing and I noticed VMWare tools will STILL NOT roll the time back on a guest that is running FAST. Which to me is completely retarted. I have some guests....6 out of 330 guests in our fleet to be exact, that have to all be within 3 seconds of each other for some terrible P.O.S. app they run in the guests. Anyway....windows time svc,etc will roll time BACK if needed so why will vmware tools not roll time back. This stinks on ice because I have to setup windows time svc to poll more frequently.

I am at the absolute latest build on 3.02 in the farm I am testing with, have all the hosts setup right for NTP, have the latest build of vmware tools and I just tested and it still will not roll time BACK. It will roll it FORWARD OK, just not BACK. I have seen my guests peridocially jump forward, especially SMP VM's (mor ethan one VCPU) and then slow up, and go fast,etc. from what I can tell, CMOS time/chip/clocking emulation is something that can be weird in VM's due to the fact that you are not clocking the guest from a real BIOS,etc and the CMOS clock chip is virtualized and created by clock ticks from the host so in some cases it is not something that is a constant.

VMWARE, If I am wrong here, please correct me and then fix the VMWare tools so it will roll time BACK if needed, I am sure it is not exactly rocket science

Thanks

[admin]

According to page 14 of http://www.vmware.com/pdf/vmware_timekeeping.pdf

VMware Tools time synchronization is designed to be a second line of defense to deal with special cases where a guest operating system's clock falls behind real time despite the built-in catchup mechanism provided in the virtual machine. It is normal for a guest's clock to be behind real time whenever the virtual machine is stopped for a while and then continues running; in particular, after a suspend/resume, snapshot, disk shrink, or VMotion operation. These are the main cases that VMware Tools time synchronization is meant to handle.

In other words, correcting fast clocks is not intended to be done via Tools, but by fixing the underlying problem. The PDF has some common problems and solutions.