On Thu, Mar 24, 2011 at 03:38:32PM -0700, Chuck Swiger wrote: > 1) Running ntpd in the Dom0/host ESX/host is very useful. Keeping good time > there means that good time will be available to all of the VMs/guests via > independent_wallclock = 0, vmx option tools.syncTime = true, etc.
This is no longer the case with Xen, at least with the domU support in the mainline Linux kernel. The effectiveness and necessity of ntpd is indistinguishable from real hardware, best as I can tell: if you don't run ntpd, your clock will drift. If you do run ntpd, everything is fine. It has no problems keeping accurate time, and the PLL stats show no differences from real hardware. It makes for perfectly capable pool servers, too. I am not 100% sure I fully understand the reasons why this is so; I nearly got into a discussion about it on IRC a few days back, but had to leave before I could look too much into it. My working hypothesis is that there's nothing magic between the domU kernel and the CPU, so clearly, whatever hardware oscillator is driving the kernel's concept of the passage of time does not suddenly become an ideal oscillator because of a hypervisor. (This is based on practical experience with Linode, Rackspace Cloud, and Slicehost, all of whom include ntpd in their base deployment images.) -rt -- Ryan Tucker <[email protected]>
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