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

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

inas mohamed wrote:

> 1. when I create a VM I am allocating memory for this one, my 
> question is : Is this memory is reserved to that VM even if it is 
> down?and if yes how can I use this memory for another VM while it is 
> down?
The memory is reserved for the VM while it is running, but is 
available if the VM is stopped. I believe that at the moment, there 
is no dynamic memory usage - ie you cannot overcommit and have memory 
automatically moved between VMs.
But there is a mechanism called ballooning which allows memory to be 
dynamically altered (at least for PV guests, not sure about HV 
guests) - so you can manually balloon down memory on one guest and 
reuse it in another. I think there is work going on with a view to 
automating this, but if you stop and think what's involved, it's a 
very non-trivial task.
> 2. If I have xen host with 12G RAM and I want to create 3 VMs on 
> that host what is the maximum value for RAM size I can use to leave 
> enough memory for the XEN host to work fine?
Depending what you are doing in the host, about 512M should be fine. 
It's worth adding a "dom0_mem=512M" statement in your bootloader* so 
that the Dom0 kernel sets it's table sizes according to the memory it 
will be using rather than the memory in themachine.
* As in "kernel  /xen-x.x.x-amd64.gz dom0_mem=512M"

> 3.If many hosts joined a pool lets say 3 hosts each with 12G RAM and 
> each one has its local SR (no shared storage). Can I create a VM 
> with more than 12G RAM and specify a sr of one host for it?
No, you cannot share ram across hosts. On a host with 12G ram, you'll 
be limited to something like 11G for a single VM - allowing 1/2G for 
the host itself, and some overhead (not sure how much) for Xen.

When setting up your guest, it is worth setting "memory" and "mexmem" 
- memory is how much the guest will be started with, mexmem is the 
upper limit it can be ballooned to.
Using the old style config files, it would be set like this - dunno about XCP :
memory  = '256'
maxmem  = '512'

--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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