Xen devel - Jul 2012 - Xenstore restart

Hi,

I am using xen-4.1-testing with Linux 3.4.3. The hotplug script
xencommons echoes that xenstore is not restartable. Can anyone tell me
the exact reason why it is not restartable. I tried to restart
xenstore by killing the xenstore and restarting it with xencommons but
after that VM creation doesn''t work. I used to do the same with the
setting <xen-3.4+Linux-2.6.18> and it used to work fine.

thanks,
Shakeel

On Sat, 2012-07-28 at 00:49 +0100, Shakeel Butt wrote:
> I am using xen-4.1-testing with Linux 3.4.3. The hotplug script
> xencommons echoes that xenstore is not restartable. Can anyone tell me
> the exact reason why it is not restartable. I tried to restart
> xenstore by killing the xenstore and restarting it with xencommons but
> after that VM creation doesn''t work.
The C xenstored has no mechanism for remembering the list of registered
watches over a restart. Things like the backend drivers in dom0 rely on
watches which are registered at start of day (with no way to reregister)
so if you restart xenstored then they stop working and you can''t start
domains etc.
>  I used to do the same with the
> setting <xen-3.4+Linux-2.6.18> and it used to work fine.
Are you sure? I don''t think this has ever worked.

It might be possible to restart the ocaml xenstored, if you arrange to
give it the appropriate command line parameters.

Ian.

> The C xenstored has no mechanism for remembering the list of registered
> watches over a restart. Things like the backend drivers in dom0 rely on
> watches which are registered at start of day (with no way to reregister)
> so if you restart xenstored then they stop working and you can''t
start
> domains etc.
Can''t I just restart the backend drivers in dom0 after restarting
xenstored. I
haven''t started any DomUs yet, So, restarting backend drivers should
not be
an issue.
>>  I used to do the same with the
>> setting <xen-3.4+Linux-2.6.18> and it used to work fine.
>
> Are you sure? I don''t think this has ever worked.
I will recheck this.

thank,
Shakeel

On Sat, 2012-07-28 at 15:09 +0100, Shakeel Butt wrote:
> > The C xenstored has no mechanism for remembering the list of
registered
> > watches over a restart. Things like the backend drivers in dom0 rely
on
> > watches which are registered at start of day (with no way to
reregister)
> > so if you restart xenstored then they stop working and you
can''t start
> > domains etc.
> 
> Can''t I just restart the backend drivers in dom0 after restarting
xenstored. I
> haven''t started any DomUs yet, So, restarting backend drivers
should not be
> an issue.
That might help, if everything were sufficiently modular. However I
think there are some watches which are registered at start of day by
core code which cannot be unloaded/reloaded. I don''t know if any of
those are critical though, the main ones which spring to mind are the
reboot control node (which isn''t critical for dom0) and the core common
backend handler (which might be possibly modular but which I have a
feeling can''t be made so in some kernels).

Ian.