RE: [Xen-devel] XCP Resource Pools and Workload Balancing Questions

[Dave Scott <Dave.Scott@xxxxxxxxxxxxx>]

Hi,

> I have some questions about resource pools and workload balancing in
> the Xen Cloud Platform:
> 
> 1) Is there anything behind the default maximum of 16 hosts? Does it
> have to do with the complexity of workload balancing for VMs or
> something else? Would I have to change a particular parameter to raise
> the limit or simply ignore it?

You can ignore it.
There's no host limit in the xapi code and there's no reason to think that the 
system will fall apart with 17 (or 18 or 19...) hosts. It's simply that our 
current testing focuses on resource pools of <= 16 hosts.

> 2) You give 64 VMs as the example limit for a resource pool. Is there
> any reason for a limit of 64 other than the complexity of workload
> balancing for VMs?

There's no VM limit either. I've run resource pools with ~1800 active VMs 
without problem (although workload balancing was disabled).

> 3) Homogeneous hosts: Is this an enforced requirement? One can imagine
> that with a large resource pool, there will be groups of machines that
> have different configurations. I wish to treat these as a single large
> resource pool and not break it into mini-pools.

This is *partially* enforced. If you attempt a 'pool.join' operation with 
heterogeneous hosts you'll get an error. You can specify '--force' to do the 
join anyway... however there's currently no CPUID masking so you might have 
problems when migrating VMs between different hosts.

> 4) Workload Balancing: Is the Analysis Engine a replaceable component?

Xapi talks to an off-host workload balancing service via a SOAP-like XML HTTP/S 
interface. Check out:
 
http://xenbits.xen.org/xapi/xen-api.hg?file/1d27457a84aa/ocaml/xapi/workload_balancing.ml

If you're interested in working in this area we should probably discuss this 
further on xen-api@lists where we do the xapi dev stuff.

Cheers,
Dave


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