[ntp:questions] Reference clock driver for /dev/rtc

unruh unruh at wormhole.physics.ubc.ca
Sat Jun 26 13:33:41 UTC 2010

On 2010-06-26, David Woolley <david at ex.djwhome.demon.invalid> wrote:
> unruh wrote:
>> Linux, depending on the setting of a flag in the adjtimex setup, sets
>> the rtc from the system time once every  11 min. . This is a disaster if
> I think you missed Dave Mills' point that ntpd does this every 60 
> minutes, so will also break mechanisms for compensating for RTC drift 
> whilst the processor is powered down.

His point seemed to be that a) He did not know aboutthe Linux 11 min
mode (a statement that may have been a rheotorical device) and b) that
ntp did it every 60 min. I do hope that ntp's mechanismcan be switched
off by some option. That 60 min mode is as silly as the 11 min mode
(except giving the rtc more time to drift if wha you want is to minimize
the rtc offset while the computer is running and rtc is never used.)
Of course this is in the context of VM using the rtc driver to read the
underlying system clock, a misuse of the rtc concept IMHO, and either
mode would be a disaster if the rtc driver on the VM actually had
permission to change the system clock on the underlying real machine.

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