[ntp:questions] Re: ntpd, boot time, and hot plugging

Brad Knowles brad at stop.mail-abuse.org
Thu Feb 3 14:56:51 UTC 2005

At 3:00 PM +0000 2005-02-03, Tom Smith wrote:

>  I know the subject has been workstations, but let's talk for a moment
>  about this religion as it concerns servers - like the ones that run
>  telephone companies, stock exchanges, and banks inside heavily
>  defended firewalls. It's the same issue, it's just that the stakes
>  are higher. The issue is how quickly can you get these
>  systems back up at boot. 15-30 seconds is a long time to wait.
>  Too long.

	With a decent drift file and using iburst throughout the server 
definitions, Steve has demonstrated that you can get this down to 
about seven seconds across a cable modem line, without any local 
Stratum 1 time servers.  This is real-world experience.

	If your servers are time-sensitive, then they should be the ones 
best able to tolerate that extra seven seconds during the startup 
phase.  The more important it is to have the time correct, the more 
important it is that you be able to tolerate short delays on startup.

	If you want to make that delay shorter, I guess you could package 
Stratum 1 refclocks with every machine.

>  We're not talking about one-shot sampling for maintaining the time,
>  so comparisons to SNTP are not helpful. We're talking about speed of
>  acquistion of an initial "good enough" time, keeping in mind that the
>  perfect is often the enemy of the good.

	Seven seconds to find "good enough" seems to be a pretty good 
balance to me.

	However, if you want to shoot yourself in the foot with a 
thermonuclear bomb, please feel free to do so.

>  The reason why so many of your constituency keep bringing this
>  subject up is that they know that ntpd needs a good (not perfect)
>  estimate of the time before it starts and that critical systems
>  can't wait for perfection to get that estimate.

	I don't know how much more perfection you want.  If you can't 
tolerate seven seconds during the startup phase, then you're using 
the wrong protocols.

	If you need a true fault-tolerant real-time system with 
resolution down to attoseconds, and those seven additional seconds 
during startup are effectively seven additional aeons for your 
application and you cannot possibly tolerate them, then you shouldn't 
be using TCP/IP, Unix, or anything else that anyone on this list 
would recognize.

	In this case, ntpd is the least of your worries.

Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.

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