Steven M. Bellovin wrote:
> Is the "real" memory allocated to Xen DomU machines paged by the Dom0
> machine?  Put another way, the machine has a certain amount of RAM.  What
> are my choices for allocation among several DomUs?  It's obviously bad to
> overcommit too much, resulting in double-paging, but am I restricted to
> the total amount of real memory in assigning memory sizes to the VMs?

I would certainly expect that trying to allocate more RAM to your VMs than is 
physically available in the actual hardware would result in severe performance 
degradation *IF* all of your VMs are active at the same time.

On the other hand, if you've got a bunch of VMs which are mostly idle and only 
handling, say, less than 10K HTTP hits per day (or emails, etc), you can 
probably get away with over-allocating RAM to your VM's.  You should 
definitely watch out for signs of contention and severe increases to server 
latency if the load picks up.  There are monitoring tools like Nagios or 
BigBrother which'll keep track of latency on a per-port/service basis which 
might be helpful...

-- 
-Chuck