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Yes, the point of a pool is to be able to move a VM between physical nodes.

You cannot span one VM across multiple physical nodes. That is not how virtualization works. For example, if you own two 8GB XCP hosts, you cannot create one big 16GB VM. It simply does not work that way. What you could do is create VMs that were smaller than the RAM size of the physical nodes. For example, you could create several 1GB VM and then move them between the XCP hosts as needed.

-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of inas mohamed
Sent: Tuesday, March 08, 2011 4:35 AM
To: xen-users@xxxxxxxxxxxxxxxxxxx; Simon Hobson
Subject: Re: [Xen-users] I need some answers please

thanks for your reply.

Can I understand from your answer that the resource pool benefit is for moving VM from one XCP host to another . And not sharing their resources so I can create VM with RAM size equal to the summation of two XCP hosts RAM size???

Thanks

--- On Tue, 3/8/11, Simon Hobson <linux@xxxxxxxxxxxxxxxx> wrote:

From: Simon Hobson <linux@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] I need some answers please
To: xen-users@xxxxxxxxxxxxxxxxxxx
Date: Tuesday, March 8, 2011, 12:13 PM

inas mohamed wrote:

> 1. But if this is the case what is the benefit of using a resource pool??

Not sure what the question is here. You have a real machine with X amount of memory. By the time you've allowed some for Xen and Dom0, you have Y left over. You can snow start guests that use up to Y memory between them. If you shut one down, you can reuse it's memory for another guest.

If things get unbalanced, you can move a guest to another host that has more free memory - and given the right setup this can be a live migration.

If you find you need to start another guest, then (depending on the systems) you may be able to squeeze enough memory by ballooning the other guests down a bit.

> 2. You said if VM is down it will release the RAM. why when I tried to find free memory on the host (using free -m) I am not be able to see more free memory? actually even the total memory is not correct.So, the correct question is how to find the total and free physical memory on XCP host??

Don't forget that the Dom0 kernel is also running as a (PV) virtual machine. It will only see the memory given to it by Xen - none of the normal GNU/Linux tools will show you real memory utilisation for the host machine.
Not sure where else to find the memory utilisation, but it is shown by xentop.

-- Simon Hobson

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RE: [Xen-users] I need some answers please