Re: [Xen-devel] "right" way to gather domU stats in xen 3 & 4?

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Sat, 26 Feb 2011, Florian Heigl wrote:
> Hi all,
> 
> I'm building a xen agent for nagios / check_mk.
> Automatic inventory of VMs and the basic up / down reporting are
> reliable now, and I'm looking at the next items on my list.

it looks like a interesting and useful project


> * Free memory. This seems easy at first, look at xm info and that's
> mostly it. I can have a different color for memory allocated to dom0
> minus the dom0 lower balloon limit, but I'll also have a check that
> will go to full alarm if anyone is crazy enough to use dom0 balloning.
> ;)
>   What I don't know is if I also need to substract something for the
> Xen heap? Long ago it used to default to 32MB i think. Can someone
> clue me in about that - is it relevant to xm info free / total mem?

libxenlight provides a function that is called libxl_get_free_memory
that returns the amount of free memory in the system.
You can call it directly (adding a libxenlight dependency to your code)
or you could simply give a look at the implementation
(tools/libxl/libxl.c:libxl_get_free_memory).
Also on hosts managed by libxenlight there is an additional xenstore
node called /local/domain/0/memory/freemem-slack that contains the
amount of memory that is going to be left free for Xen.

In case you are wondering xen 4.1 is going to ship with two toolstacks:
the old xend and a new one that is a library called libxenlight plus a
minimal C utility called xl to invoke the library functions.
xl/libxenlight are recommended over xend.


> * per domU I also wanna look at memory statistics.
>   -  one thing is: mem vs. mem-max to show balloning.
>   -  the other thing is tmem: i don't know if i should spend the time
> getting it right as I start getting the impression that since it was
> added by Dan and now tmem2 was added, two-and-a-half years went down
> where it's considered working implemented none bothers to make it work
> for everyone. i.e. the recent directed that the direct ballooning
> daemon was just a lab exercise ;) If you know of any people that
> successfully run xen with tmem2 and such, I'd love to work with them
> to build the nagios-sy statistics .Otherwise I'll save myself the
> headaches.
> 
> * per domU cpu percent (to show how much of the dom0 power the vm is
> consuming)...
> 
> 
> Speed issues:
> Usually checks in check_mk are fired off every minute, so it would be
> good if I can directly via xenstore to collect and report my data
> within 1-2 seconds or less. Speed seems to be an issue I have to worry
> about - on my "top of the shelf" xen host it will take around
> 0.6seconds to query a meager 5 VMs.
> That's just a 1.5GHz VIA box, but I'll have to see how long it takes
> for 100 VMs or more.

Xenstore can become very busy on systems with many VMs running.


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