| Re: [chrony-dev] can chrony adjust H/W clock too ? |
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On Wed, 21 Apr 2010, J. Bakshi wrote:
On Wed, 21 Apr 2010 10:55:07 +0100
Ed W <lists@xxxxxxxxxxxxxx> wrote:
The point was not to use both hwclock AND chrony to condition the RTC
since they are both trying to do the same job independently and will
fight...
This is the very point which I was asking for. If chrony itself takes care off rtc then there is no need to use hwclock individually through cron.
Remember, linux never uses the rtc while it is running. All times are
determined by the software system clock ( which uses hpet, or counting the
cpu cycles, of...). It is only on bootup that the rtc is used to set that
system clock. Thus the accuracy of the rtc is of no consequence except on
bootup. And even then chrony can rapidly poll the ntp servers and set the
system clock properly. What chrony can do ( using the -s option) is
a) to set the system clock from the rtc on bootup, correcting it for the
offset and drift determined during the last time chrony was run, and b)
measure the offset and drift of the rtc.
One of the key places wehre chrony can be very useful is if you dual boot
windows and Linux. Windows wants the clock on local time. Linux on utc. So you
can set the rtc to local time, so when windows boots up, its time is
(approximately) right, and let chrony set the system clock from rtc. Chrony
will have determined that the rtc is say 8 hours off (PST) and compensate and
set the system clock properly (to UTC). Of course this still causes trouble at
the conversion to daylight savings time, but then windows always has that
problem. It just makes it a (relatively minor) problem for linux as well, as
the time will initially be an hour out. chrony will rapidly correct that, and
determine that now the rtc is out by 7 hours, rather than 8.
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