[Users] Re: Bug#636463: vzctl: /etc/vz/dists/scripts/postcreate.sh randomises run-times in /etc/crontab, but not cron.d

Ola Lundqvist ola at inguza.com
Wed Aug 10 00:47:02 EDT 2011

Hi Tim

On Tue, Aug 09, 2011 at 10:27:15AM +0100, Tim Small wrote:
> cc'ing openvz users mailing list, as this has been discussed there recently.
> On 04/08/11 06:36, Ola Lundqvist wrote:
> > This is certainly an interesting problem. The patch is a good start but
> > as we can foresee problems already now we need to find out a solution
> > to that before it is applied.
> >   
> I agree.
> > There is also one other problem. It is just a very limited number of
> > files that are actually installed during the initial install when
> > postcreate.sh is executing. Actually on my computer it was only anacron
> > that hold a file in that directory (that I have not created myself or installed
> > post initial install).
> >   
> This I suppose depends on the particular site's style of working - if
> you have "fat" VZ templates it's likely to be OK?  Or maybe I've
> misunderstood when this script gets called.

Yes you are right. For "fat" VZ templates it would be more likely to
be ok.

> > All other applications will have the same problem as before.
> >
> > I can see two solutions:
> > 1) postcreate.sh go through only the files that are actually created
> >  on initial install and touch them in a similar way as your patch.
> > 2) A new tool is introduced that should be run by the system administrator.
> >
> > Do you have any opinion about this?
> >   
> Difficult to see the best answer here, and indeed it's a problem which
> is not OpenVZ-specific of course - it's equally valid across any
> virtualised instance of Debian - which is becoming more and more the
> normal way to deploy servers.

Even across any virtualize unix system using cron. :-)

> For that matter, you can definitely see when cron.daily fires off by
> default at 6.25 every morning on this traffic graph for
> ftp.uk.debian.org - http://www.hands.com/mrtg/free.eth0.html - and I've
> experienced problems with jobs like backups running all-at-the-same-time
> via default crontab setups on networks of non-virtualised servers.

I have experienced this myself so I fully trust you. I mean there is a reason
why the cron config is rewritten.
> As virtualised instances of Debian become more and more common, maybe
> it's something that needs to be solved for the general case at package
> install time?  Crontabs installed by packages could have meta-info
> included to indicate whether the crontab entries should be perturbed,
> and if-so, by how much?

That would be a way. Or even the package could randomize it on itself.
I mean the same thing happens on physical servers as well if you have 
many of them.

> This is obviously not a small change, but having thought about it for a
> while, other possible approaches probably have the possibility of
> breaking the packages which they alter, and not fixing things in the
> general case?
> I suppose it's something that could be prototyped in OpenVZ (e.g. by
> shipping the meta-data separately from the packages initially, and only
> perturbing crontabs which are "opt-in") - in our experience it was only
> a small subset of crontabs which caused the problem (mysql and logcheck
> were the biggest ones, if I remember correctly).

I'm leaning on submitting wishlist bug requests on packages that can
cause a load on external functions (network or similar) to randomize
their cron file.

Best regards,

// Ola

> Tim.
> p.s. a quick bit of research seems to show up the problem coming up
> again and again...
> https://bugs.launchpad.net/debian/+source/cron/+bug/672303
> http://projects.puppetlabs.com/projects/1/wiki/Cron_Patterns
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=373152
> etc. etc.
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