Re: [Xen-users] Few Questions / Application of Xen

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > Currently you need to do this manually. More powerful automatic load
> > balancers are likely to appear on the market at some point, as an
> > additional management product.
>
> Ok, so you're saying there's currently no solution at all ?
> Any opensource project (even beta) that you are aware of ?

http://www.enomalism.com - an opensource Xen management tool.  Not released 
yet, not clear if it does load balancing anyhow.

http://www.xensource.com - to be releasing a product called "Optimizer", will 
include management functions.  I don't know about load balancing for this 
either.

> Basically, I will need this feature as part of my whole project....so
> I'm planning to code my own Xen load balancer... but I would prefer to
> help an existing project rather than starting from scratch my own
> solution....

Indeed.

> > Two aspects: for a planned shutdown you'd want the management software to
> > live migrate domains off a server you were planning to take down. You'd
> > also like it to detect a crash, and reboot the domains on other hosts
> > (and lock out the downed server from shared storage so that if it comes
> > back on the network any remaining domain images it's running can't
> > confuse the situation)
>
> Ok, actually, if you can load balance, it's pretty easy to detect that
> a machine is not responding and correctly manage the failure by
> sharing the remaining load amongst machines that are alive.
> So my question was stupid, sorry :)

Well, if a machine has just been partitioned from the network (and not 
actually brought down) you have to make sure the domains on it *really* die, 
else they'll corrupt storage if that machine comes back after the domains 
have been evacuated.

> It's pretty easy to script such a thing when you plan the RAM modification.
> Like getting the virtual machine to 128 Megs -> 256 Megs...
>
> But for programs that are quite "memory volatile" it's rather hard...
> for example a game server that is growing from 30 Megs to 80 Megs and
> that can't stop doing that depending on the number of users....
>
> To add some complexity my main problem is to detect how much RAM is
> really needed by the virtual machine, because of the way Linux is
> often caching objects in memory, I'm getting a little bit confused...

You need to the other domUs to report their swap usage, or something like 
that.  vmstat ?

Then balloon based on that (or on their non-cache memory usage, as reported by 
"free", or whatever).  A network monitoring tool would also work.

Cheers,
Mark

-- 
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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