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Re: [Xen-users] Virtual machine(s) performance questions

On Tue, Jun 16, 2009 at 9:01 AM, Jason Brooks<jason@xxxxxxxxxxxxxxx> wrote:
> Hello,
> I am getting ready to convert a server from Uni-processor to xen.  I am
> curious about a few things:

The generic answer would be "it depends on your requirement. The best
setup for one system might not be the best for another".

> 1) Is there an ideal disk setup for virtual machines?  There are many
> combinations I could try.  Here are some examples I have seen.  Am I missing
> something?  I assume the following in order of highest to lowest
> performance.

My favorite would be using LVM on dom0 (if storage is on local disk).
If using SAN, I'd use separate LUN for each domU. That should provide
some balance between performance and managability.

> 2) It's apparent I should not assign more memory total to all domU and dom0
> vm's.  Are there proportions I can optimize with?  How about the hypervisor:
> Should I allocate more memory to it?

I'd generally allocate 500MB - 1GB memory for dom0. The rest can be
used for domUs as necessary.

> 3) It seems apparent that I shouldn't have more vm's and processors than
> physically exist.  Is this true?

Not really. That depends mostly on what your load is like.
Xen should be smart enough to allocate CPUs dynamically as necessary.
So if your general load is low enough, you can have (for example) 16
domUs on a server, 1 vcpu each, on a server with only 4 physical CPU
cores. In general however :
- you should dedicate 1 core for dom0
- you should not allocate vcpus for one domU greater than the amount
of available physical core

>  How should I compute the total physical
> system load?  Under a non-vm, I can look at the load averages of a system to
> determine how "loaded" a system is.  Is there a way to do this with all
> vm's?  xentop seems limited...

You can combine xentop with snmp and cacti. To determine how "loaded"
(cpu-wise) your system is, you can look at each dom's CPU time.

> 4) Does sharing filesystems between vm's work better with nfs or GFS?

nfs is much simpler to create and manage, while GFS/OCFS2 might
provide better performance at the cost of increased complexity.


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