[Linux-HA] Announcement: New book about Linux-HA Version 2

[Dejan Muhamedagic]

Hi,

On Wed, Jun 04, 2008 at 01:16:31AM +0200, Lars Marowsky-Bree wrote:
> On 2008-06-03T16:35:22, "Hildebrand, Nils, 232" <Nils.Hildebrand at bamf.bund.de> wrote:
> 
> > > - [Quality not important for hardware?]
> > I think the truth is somewhere in between. If you have a cluster it is
> > not that important if a node goes down (depending on how available your
> > services have to be).
> 
> Right, but telling customers to save on hardware because they are
> getting clustering is, uhm, not something I'd advocate ;-)

Indeed. Clusters are not a replacement for good hardware. A
perfect cluster is one which is never exercised (unless when
testing) ;-)

[snip]
> > > - [time is important - but only for logs]
> > Why is there a heartbeat-message "time has jumped back - compensating"?
> 
> That doesn't track whether time is set correctly, but whether the system
> time as reported is monotonously increasing. And if it is not, it is
> compensated for.
> 
> That can happen when the clock is adjusted abruptly, ie it is a possible
> side effect of running ntpdate via cron too. Not all applications can
> compensate this, some will exhibit rather strange behaviour because they
> are more trusting than our rather paranoid libraries ;-)
> 
> xntpd avoids this by smoothing all jumps and only making minimal
> adjustments. The right combination is to use ntpdate once at system boot
> and then run xntpd (like the xntp init script does.)

Some virtual environments are notoriously bad at keeping time
(vmware for example). I think that sometimes ntp can't prevent
clock from jigger.

Thanks,

Dejan