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[Xen-devel] RAM allocation and miscellaneous questions

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Subject: [Xen-devel] RAM allocation and miscellaneous questions
From: Paul Dorman <pauld@xxxxxxxxx>
Date: Wed, 29 Sep 2004 14:57:48 +1200
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Hi everyone,

I'm running 10 VMs now quite happily, using LVM2 with distinct storage areas 
for each VM filesystem.

I suspect I'll be able to run around 30 VMs on the dual xeon 2.8GHz machines 
successfully, but I'm wondering how to manage the RAM allocations for each. I 
calculate that each VM will have around 110MB RAM if I run that many, 
including a healthy amount of RAM for Dom-0. I am wondering how this can be 
enough for LAMP VMs. Some of the systems I'm running now don't seem to 
perform very well with less than 400MB of RAM, and I can only really do that 
if some kind soul fixes Xen so I can use the full 12GB capacity of my Xen 
servers (they have 4GB now -- no good wasting money!).

This is probably a stupid question, but can I allocate more RAM to each VM 
than arithmetic would suggest? Is there some kind of compression or COW that 
goes on in the Xen memory management code?

Would it be best to run one replicated MySQL server (or several with different 
versions) loaded into a VM on each physical machine, and have all the other 
machines use that over virtual network connections? I suppose this would be 
perfectly okay, and I could do the same with Apache too I guess. But as I am 
going to be migrating the sites from many real machines into Xen, splitting 
them up into individual site machines as I go (at the moment our servers can 
run many sites), it would take a lot of work to change their configuration so 
that they connected to a networked copy of MySQL or Apache rather than a 
local one.

Another option, and I'm thinking you are all going to say 'go for that one!', 
would be to use 64bit machines for our main cluster, perhaps just using the 
two dual Xeon machines I have now for development purposes. That way I could 
use lots of RAM and presumably load up even more VMs into each. If I go down 
this road, I'm wondering what kind of hardware I should get. Does Xen work 
well with Opterons? I'm an AMD fan and would be very happy to use them if I 
went for this option. I'm after inexpensive commodity servers, so if anyone 
has any recommendations (especially if you are in NZ or Australia), I'm all 

May I ask how much RAM I could use with a dual Opteron system in a sensible 
Xen configuration? What's been tested?

One final question: if I were to ask the directors of the company if they 
would be interested in sponsoring big memory on Xeon-based machines, what 
kind of commitment would we be talking about? It could be cheaper to do this 
than to buy a cluster of dual Opteron machines! I do realise though that the 
true impediment will probably be the amount of time you developers have, so 
I'll brace myself for disappointment! :o) 

Thanks everyone for your valuable time.


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