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I have a VM with Oracle Linux 5.5 where the BIOS clock (also known as hardware clock or RTC) is wrong after boot.

What I did:

  • boot the guest Linux VM with the boot parameter init=/bin/sh
  • check time with hwclock -r or simply date as the sys time is initialized from RTC

Result:

Time is wrong, about a day in past.

According to references below, Hyper-V sets the VM RTC on boot to current host time.

But in my case this is not happening. Why?

It seems the last set clock value is saved/remembered instead of setting the current correct time. More exactly:

  • if I store into RTC a date in past (like Jan 1), the on next boot the value will read as Apr 16 (today is Apr 18)
  • if I store into RTC a date in future (like Jul 7), the on next boot the value will read as Jul 7

Is maybe the initramdisk interfering? I tried to bypass it too, but it seems it is required to mount the root fs.

The VM is running Oracle Linux 5.5 (based on Redhat Enterprise Linux 5.5).

References:

Microsoft blog "Time Synchronization in Hyper-V" https://learn.microsoft.com/en-us/archive/blogs/virtual_pc_guy/time-synchronization-in-hyper-v

whenever you start a virtual machine we take the time from the management operating system and put this into the real-time clock of the virtual machine.

serverfault answer at Completely disable Date and Time sync in a Hyper-V VM

Hyper-V guests always synchronize time with the host on BIOS level during boot.

David Balažic
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I assume that your Hyper-V platform is a lot more current than your VM with Oracle / RHEL 5.5 which is an ancient legacy release from early 2010. Since RHEL 5 has already been End of life for several years it is probably no longer a supported OS for current Hyper-V installation.

For RHEL 5.5 there is no built-in support for Linux integration services (LIS) but you could/can install it manually. Not sure if that will help with your clock issue though because:

RHEL 5.5 has no Hyper V clock source driver in the kernels, that was introduced only later, in RHEL 5.11 from 2014.

What I remember is that we needed to use NTP to sync RHEL 5 clocks in the VM rather than being able to rely on the Hyper-V hypervisor to keep the guest time synchronised.

HBruijn
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