Re: [Xen-users] cron and domU's

[Javier Guerra <javier@xxxxxxxxxxx>]

On Mon, Dec 14, 2009 at 11:30 AM, Israel Garcia <igalvarez@xxxxxxxxx> wrote:
> Hi, I'm getting bad performance on domUs cause all dom0 and domUs
> crontab's are setup at same hour 6,25AM. I want to ask you if there's
> something I can use to centralize setup of all crones from domU
> instead of setting up every domU's cron (to run in different hours)
> manually?

I see at least two levels of answering:

A) yes, there might be some kind of centralized contrab config tool.
This isn't exclusive to Xen, any datacenter with several similar
machines must setup backups and other heavy processes to avoid
fighting for some scarce resource (like a fileserver, or a shared
device).  At the very least, there's a webmin module for 'cluster cron
jobs' but this might be exactly the opposite (it seems to try to
execute all at the same time)

B) do you mean running cron in Dom0 instead of on all DomU's?

  B.1) on one side it could be reasonable, after all that's
(conceptually) how backup servers do: they have their own scheduler
and 'pull' the data from the running servers.  still, i wouldn't run
it in Dom0, but on the machine (virtual or real) that manages the
resource.  IOW, not a 'cron server', but a schedule for backups on the
backup server, a schedule for updating mirrors on some mirror server,
a schedule for OS updates on some local updates mirror, etc.

  B.2) if you mean having all that's cron-able managed from Dom0; then
no, that's not how you should do it.  First of all, that would mean
that this single machine can execute any administration command
directly on every machine, creating a HUGE security issue.  And second
because then you go in a slippery slope of putting everything that's
centralizable on Dom0 (first the cron, then logfiles, oh and logrotate
too, the OS updates, why not the users logins, later the SELinux
profiles, etc).  soon the DomUs are limited to a single process each,
and a big, fat Dom0.  Much better would be just forget about
virtualization, and simply run several processes on a single OS
instance.  after all, that's what shared hosting services do



-- 
Javier

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