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

Tom Smith smith at cag.lkg.hp.com
Thu Feb 3 16:06:42 UTC 2005


Brad Knowles wrote:
 > 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 ...

Precisely. The decent drift file is a problem. It sometimes doesn't
exist after a large initial offset has been turned over to ntpd.
Now, if ntpd all by itself did a quick acquisition, didn't
count that initial clock setting in any way into the frequency
correction, and blocked the startup script progress until that
was complete and it was safe to proceed with starting the
time-sensitive stuff, all would be well with the world.
If I've missed how that happens, I apologize.

 >     If your servers are time-sensitive, then they should be the ones
 > best able to tolerate that extra seven seconds during the startup
 > phase.

You should discuss that with a bank or stock exchange that
is losing millions in transactions during those seconds
or with public utility that is paying the government
penalties for downtime. :-)

 >  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.

Well, no. As David pointed out in his posting, all engineering
is a matter of tradeoffs. For many users, the tradeoff needs
to be 'Get these applications up fast on a "good enough"
time and refine the time (and frequency) in the background.'

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

Perhaps it is. For you. If it's seven seconds.

 >     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.

I don't want perfection at all. That's the point. ntpd gets it as right
as it needs to be. It just has to have something reasonable
to work with when it starts.



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