Proposal: use host based time sync instead of systemd-timesyncd by default

[CecileRobertMichon]

Follow up from #535, we should look into replacing systemd-timesyncd time sync with host based.

References:
https://help.ubuntu.com/lts/serverguide/NTP.html
https://docs.microsoft.com/en-us/azure/virtual-machines/linux/time-sync
Azure/aks-engine#2552

We can also consider enabling chrony.

These changes will most likely live in the image-builder but tracking here since the image would be consumed by capz.

[CecileRobertMichon]

from @ericsuhong's comment in the aks-engine issue above, "I found out that disabling NTP is not enough for enabling host-only sync mode. If you disable NTP, VMs till only sync from host on when VM is paused or reboot (got a confirmation from Azure Linux team). Chrony is the only way to sync time from host using PTP source."

[randomvariable]

Maybe for most complete coverage, and this would also cover vSphere environments where PTP might be enabled. Needs some testing as I don't know if ptp4l works without PTP packets arriving on a network interface:

• We additionally install and enable by default ptp4l as per
  Enable the ptp4l service https://docs.fedoraproject.org/en-US/fedora/rawhide/system-administrators-guide/servers/Configuring_PTP_Using_ptp4l/ for all images, such that if PTP is available in the environment, it would be automatically used. If it is not available in the environment, it will do nothing.

• For Azure, we provide templates with ntp servers set to 127.0.1.0 which will effectively disable NTP time sync. We would need to see if chronyd and systemd-timesync respect that value.

[randomvariable]

Chatting @nader-ziada , I hadn't realised Ubuntu switched to chrony in 18.04, so I think we're good to go with chrony for the major distros.

[nader-ziada]

/assign

[nader-ziada]

created PR in image builder repo kubernetes-sigs/image-builder#240

[nader-ziada]

PR in image-builder repo merged

[ericsuhong]

@nader-ziada Does this change mean that all default 18.04 Azure Ubuntu VMs will come with chrony enabled by default? I am wondering what would it take for us to start using this feature.

[nader-ziada]

yes, the capi 18.04 Azure Ubuntu VM will come with chrony enabled by default, but I believe we have not yet published such an image to the Azure Marketplace

[nader-ziada]

the linked PR ads chrony to the image, closing this

/close

[k8s-ci-robot]

@nader-ziada: Closing this issue.

In response to this:

the linked PR ads chrony to the image, closing this

/close

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