On a Debian server, I'm having problem with hwclock:

$ hwclock --show 
hwclock: Cannot access the Hardware Clock via any known method.
hwclock: Use the --debug option to see the details of our search for an access method.

System runs on backports kernel Debian 4.9.18-1~bpo8+1 (2017-04-10).

Here's debug output:

$ hwclock --debug
hwclock from util-linux 2.25.2
hwclock: cannot open /dev/rtc: Device or resource busy
No usable clock interface found.
hwclock: Cannot access the Hardware Clock via any known method.


$ cat /sys/devices/system/clocksource/clocksource0/current_clocksource

Finally, rtc device exists:

$ ls -l /dev/rtc*
lrwxrwxrwx 1 root root      4 Apr 29 16:41 /dev/rtc -> rtc0
crw------- 1 root root 253, 0 Apr 29 16:41 /dev/rtc0
  • Have you tried running these commands as root/via sudo? – Paul Haldane May 24 '17 at 13:02
  • @PaulHaldane Yes, of course. I have 3 identical servers, only on one of them hwclock doesn't work. – Tombart May 24 '17 at 13:04

The problem seems to be connected to clocksource, check available_clocksource:

$ cat /sys/devices/system/clocksource/clocksource0/available_clocksource
tsc hpet acpi_pm

Based on answer to "What does “clocksource tsc unstable” mean?" it looks like TSC (Time Stamp Counter) is not very reliable.

By updating /etc/default/grub you can switch to another available clocksource. I'm using HPET.


and updating grub:


After reboot hwclock works again (just reboot without any modification might have helped as well - it might be just question for how long).

NOTE: Optimal clocksource might depend on your motherboard and CPU model.

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run on a virtual machine.

su - //afswitch root.run hwclock

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  • 2
    This needs more explanation. – Sven Mar 24 '18 at 13:48

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