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Re: [rhelv5-list] Xen guests and time differences



Matthias Saou wrote :

> On some RHEL5 host/guest Xen setups, I've seen time differences of up
> to 106s after 2 weeks uptime.
> 
> Right now I've got a Dell PowerEdge 750 running 2.6.18-8.1.4.el5xen and
> running ntpd, with its time perfectly synchronized, acting as the Xen
> host.
> 
> This host has two guests running 2.6.18-8.1.10.el5xen, which this
> morning had 80s and 106s difference reported by running ntpdate. This
> is pretty bad, and according to the Xen documentation, something which
> shouldn't happen, ever!
> 
> (In http://xen.epiuse.com/xen-faq.txt see the "Xen time" section)
> 
> What I did in the first guest is to run this :
> echo 1 >/proc/sys/xeno/independent_wallclock
> Ran ntpdate and enabled ntpd, and now its time is fine.
> 
> The second guest still has its time always 106s off :
> # ntpdate ntp
>  1 Oct 12:40:27 ntpdate[371]: step time server x offset -106.553042 sec
> 
> Is there any known bug in RHEL5's Xen which explains this? What
> solutions do I have apart from changing 'independent_wallclock' on all
> my Xen guests and syncing their time as if they were real physical
> servers?

No answers, no solutions it seems, and quite a nasty bug in enterprise
environments, ouch!

I've decided to "echo 1 >/proc/sys/xeno/independent_wallclock" on all
my Xen guests and make them sync their clock using ntp themselves.
Useless added complexity since they're supposed to keep the time of the
host, but as this is not happening, I needed a solution.

I also just noticed that on 32bit x86 guests, "hwclock --systohc"
fails silently, whereas on 64bit x86_64 guests it complains about not
being able to access the hardware clock via any known method... maybe a
clue, as I've only really investigated the problem on 32bit guests, so
maybe the 64bit ones weren't affected, but I didn't check prior to the
change.

So at least to all 32bit Xen users out there : BEWARE OF SYSTEM TIME
PROBLEMS IN YOUR GUESTS!

Matthias

-- 
Clean custom Red Hat Linux rpm packages : http://freshrpms.net/
Fedora release 7 (Moonshine) - Linux kernel 2.6.22.6-81.fc7
Load : 0.28 0.29 0.28


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