Re: [Xen-users] xend, xenstored, xenconsoled restart

[Tim Post <echo@xxxxxxxxxxxx>]

On Tue, 2008-10-21 at 23:50 +0200, Ferenc Wagner wrote:
> Hi,
> 
> I'm looking for some definite info about when it's allowed to restart
> the various Xen daemons (xend, xenstored and xenconsoled).
> 
> Up to now I worked with the assumption that I can restart them while
> guest domains are running on the host and there's no harm done.
> Indeed there wasn't, until now, when on restart one of my domains
> became Domain-Unnamed (but kept on working), while the others
> disappeared and didn't ping anymore...
> 
> (I've got the domain info dump from xm dmesg and the xend logs if
> anybody can debug this.  I'm using Xen 3.2.1.)

One thing you never (ever) want to re-start is xenstored. Xend however
can usually be re-started without issue.

Think of xenstored as something like "/proc as a service". Xen itself
does not care about the name of your guest, it only needs its ID, flags,
etc. If you look in /usr/include/xen/domctl.h , in particular the type
xc_domaininfo_t , you'll see how much the userland tools rely on
xenstore .. not just for storage, but watches. This keeps the bloat out
of Xen itself and in userspace where it belongs. That's one of the
reasons why Xen is a well designed modern highly efficient microkernel.

To make the store persistent, many small writes/reads to dom-0's disk
would be needed and would be a disaster. That's why its all done in
memory. Nevermind i/o wait and latency :) As far as I know, there is no
way to export / import the contents of the store on a graceful re-start.
Trying to do the same while handling a segmentation fault would be at
(best) questionable.

In all reality, I have seen xend crash, but never xenstored. The only
practical reason for including stuff to dump and re-import its contents
would be a graceful restart .. but why would you re-start it if its
running fine? :)

Cheers,
--Tim



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