On Wed, 2008-10-22 at 15:33 +0800, Tim Post wrote:
> 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? :)
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