On 09/05/2011 08:57, Stan Hoeppner wrote:
On 5/9/2011 1:31 AM, Per Jessen wrote:
Stan Hoeppner wrote:
This is not correct. You're assuming that ntpd doesn't perform sanity checks on the system time when the daemon starts, which is not the case.
The sanity check may be disabled with -g in which case using ntpdate/sntp/ntpd -q at start up becomes pointless.
'ntpdate -q' has always been 'pointless', unless you just want to look at the offset without modifying the clock. I do so on occasion to see how accurately my local ntp server is keeping time. For instance:
Can I suggest that the OP also consider Chrony for timekeeping needs?
Chrony will generally sync faster than ntp and additionally will manage the setting of the clock at startup. Even nicer it will condition the RTC clock and so for example if your RTC is drifting (it will) then chrony tries to maintain a drift estimate and set the initial time to a sensible offset from the RTC (nice!)
Chrony just made a new release a few days ago and it has a bunch of neat features for those who want to get excited about excessively accurate clocks
I acquired 'special' permission many years ago to use a few stratum 1 USNO servers mostly because at that time I lived in a city where one of them is located, and because I only have one client querying infrequently. USNO is the official time keeper for the US Military and the US Government, including ships at sea via GPS. USNO has the most accurate timekeeping devices on the planet--atomic clocks. Most (if not all) of the stratum 2 servers in the US query the USNO stratum 1 servers.
Querying an NTP stratum 1 server over the internet will likely leave you with less than millisec accuracy. ie the original is accurate, but the limitations of syncing over the internet are significant
Compared with a cheap GPS attached to your machine which should get below ms accuracy and perhaps even below the 100us mark
So, in practice it's fairly irrelevant to be hooked to a stratum 1 for most purposes and if you really want to get obsessed about accurate time (I'm going through this obsession phase right now...) then just get a local GPS attached to your machine...
I saw some analysis from the current lead Chrony developer comparing time offsets of a bunch of public timeservers and the resulting analysis seems to be that there is quite some significant skew, even on stratum 1 machines... ie you can easily do better at home with a GPS than using a public stratume 1... Curious huh
Good luck
Ed W