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Re: [Xen-devel] Basic xenstore questions (building a watchdog)

> I'm looking at building a xenstore-based watchdog, as described at
> http://lists.xensource.com/archives/html/xen-users/2005-07/msg00597.html

Sounds cool!

I knocked up a little prototype of one some time last year.  The Xend code was 
surprisingly tricky (couldn't get rebooting the domain to behave right, 
although I didn't try very hard).  You'd be welcome to the kernel code if you 
want to take a look, although it's very simple - the hacks required to the 
softdog device in Linux were minimal.

I'd suggest as an alternative to using the store, you might like to think 
about adding watchdog functionality to Xen itself...  The advantage here is 
that it also allows you to watchdog dom0, without requiring watchdog 
hardware.  Timing out dom0 would reboot the system, a domU would just result 
in its destruction (and the tools recreating it).

Just a thought ;-)


> However, being somewhat new to xenstore, I'd appreciate some pointers.
> - What portions of the xenstore namespace should I use? I'm looking for
> at least two settings writable by the DomUs: A flag to enable/disable
> the watchdog for a given domain (ideally, this would be specific to an
> individual run of the instance in question and not persisted across
> runs), and a counter via the modification of which the watchdog may be pet.
>    http://wiki.xensource.com/xenwiki/XenStoreReference indicates that
> /tool is likely an appropriate region (perhaps I should create
> /tool/watchdog/<vmid>/{enabled,counter}?), but this doesn't provide
> guidance as to how I can avoid persistance (if this is possible).
> Alternately, are vm GUIDs unique on a per-invocation basis? If so,
> /tool/watchdog/vmid/enabled could enable the watchdog only when it
> contains the current vm's GUID (preventing it from working across
> restarts). (Given some basic playing around with xenstore-list,
> xenstore-read and friends from within a DomU, however, it looks like I
> can't read contents of /vm from the DomU -- so am I actually able to
> find my own GUID such as to be able to use it in this way? For that
> matter, how can a DomU find its own vmid?)
> - Are the xend.xenstore classes intended exclusively for xend's use, or
> is it acceptable for them to be used by 3rd-party software as well?
> - Any example code I'd be well-advised to look at?
> Thanks!
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Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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