Re: [Xen-users] Xen common hardware configuration

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Marin Cosmin <marincosmin2001@xxxxxxxxx> wrote:

> Could somebody point me what is/are most common hardware configurations for a 
> server running Xen with a bunch of domains with respect to memory, CPU, and 
> CPU cache ? I am interested in finding out whether it would be possible to do 
> some optimizations with respect to memory that is allocated to a particular 
> domain.

I doubt if there is such a list - there are so many variations of workload that 
I doubt there are many systems that share anything in common with others.

In terms of memory (and disk), that's the easiest to answer - you need enough 
for Dom0, plus enough for each of your DomUs, and a bit for overheads. So if 
you know how much each DomU needs, add them all up, add enough for the Dom0, 
and round up - that's your *minimum*. If you don't know what your workloads 
will be in the future, then you stick your finger in the air and guess ;-) OK, 
that's an exaggeration - you do the totting up for what you do know, and then 
you have to guestimate how much it will increase over the period you need to 
provision for.

You can complicate thing with ballooning. I tend to set guests up so the 
configured memory is higher than what they normally run with - eg a guest might 
be configured to have (say) 4G max mem, but start with that ballooned down to 
3G, giving me a bit more flexibility if I need to adjust a running system.

AIUI there are methods for having dynamic memory allocations, but unless you 
**REALLY** understand your systems and how they behave, then there is great 
scope for creating trouble if several systems all want this dynamically shared 
memory at the same time !

Disk space is really just the same - and again, beware of dynamic disks 
(auto-expanding disks). Many of the problems I've witnessed with virtualised 
environments (on Windows systems managed by colleagues) are due to expending 
disks, overcommitted space, and guests suddenly deciding they need more space 
until the host runs out.


CPU isn't that much different - work out how many cores each guest needs in 
order to provide the required performance under peak load, and add them all up.
There's probably much more scope for applying diversity of demand in that CPU 
cores are readily shared unlike memory and disk. But again, you need to 
understand your workloads - so no use sharing (say) 4 cores between two guests 
that need 4 cores each if they are going to need 4 cores each at the same time 
as each other, but it is OK if you know that one is going to be idle(ish) when 
the other is busy.


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