Terrible Clock Skew

Chuck Swiger cswiger at mac.com
Wed Dec 8 21:40:03 UTC 2010

Hi, Dave--

On Dec 8, 2010, at 12:01 PM, Dave Cundiff wrote:
> Hi everyone,
> I posted this to the forum as well but figured I'd try here since the
> same people might not subscribe to both.

I'm not sure which forum you're mentioning here, although it proves you right about the conclusion you'd made here.  :-)

> I've been experiencing some terrible clock skew and just can't figure
> it out. By terrible I mean I'm losing 30 minutes a day. The loss only
> occurs when I bring the system under heavy load. The load is multiple
> Rsync backups to a ZFS pool(with gzip compression) backed by a 16 disk
> Raid50. I'm using a hardware Raid controller for battery backed write
> caching.

That kind of slew rate is well past what ntpd can deal with; it suggests possibly that you're losing timer interrupts, or maybe that the "time of day" clock on the motherboard is broken or dead.  Output of "vmstat -i" might be informative for the former, for the latter check whether the BIOS can keep the clock sane while in the BIOS menu-- there should be a small Li battery on the motherboard which might be replaceable.

You might also double-check that you've got the latest BIOS version update, but that's less likely to help.

For a workaround, you can fire off "ntpdate -b" from cron every 5 minutes or similar to keep the clock from drifting too far.


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