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Cassandra user here in AWS. Clock drift is a big problem on VMs. NTP is not aggressive enough in these environments to keep clocks relatively in sync. We regula
by nvarsj 9y ago
Cassandra user here in AWS. Clock drift is a big problem on VMs. NTP is not aggressive enough in these environments to keep clocks relatively in sync. We regularly had several hundred milli drifts between nodes. As cassandra is extremely clock sensitive, this is a big problem. We ended up using chrony with very aggressive settings to keep things in the sub-ms range for the most part. But it's still possible to get "hiccups" where time will skip. Especially if you reboot a VM.
Vanilla ntp makes assumptions about the hardware clock (that drift is stable) that don't apply to virtualised clocks. Using tsc clocksource may help as well.
- newman314 9y agoInteresting. I wonder if anyone has documented any best practices for timekeeping in VMs. VMware has this but it does not appear to have been updated in a while. https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006427 https://kb.vmware.com/selfservice/microsites/search.do?langu...
- andy_ppp 9y agoWhy does VM ware emulate the hardware clock rather than giving (possibly slightly debounced) access to the real system clock?
- koffiezet 9y agoI suppose for vMotion purposes. A VM is not tied to a physical machine.
- gerdesj 9y agoI run a lot of VMware: * Set the esxis to have five external sources * Search fwenable-ntpd (https://www.v-front.de/2012/01/howto-use-esxi-5-as-ntp-server-or-howto.html https://www.v-front.de/2012/01/howto-use-esxi-5-as-ntp-serve...) and download the .vib (do a security audit on it - its a zip file I think - to ensure it is what you think it is). Install the .vib which simple adds a ntp daemon option to the firewall ports. This works on v6.5 * Run ntpd on Linux VMs, pointed at the hosts with the local clock fudge as a fallback * For Windows VMs in a domain, set the AD DC with PDC emulator role to sync its clock to the host via the VM guest tools, leave the rest alone * On your monitoring system make sure that it has an independent list of five sources and use plugins like ntp-peer for ntpds and ntp-time for Windows (Nagios/Icinga etc) With the above recipe, ntpq -p <host> shows offsets less than 1 ms across the board for ntpds after stabilising.
- newman314 9y agoHijacking my own thread: I don't suppose anyone knows how to make a Windows NTP server permit queries? Googling does not seem to reveal anything insightful. I know how to do this for ntpd but am stuck with dealing with a Windows NTP server right now.