Re: [Xen-users] How does XEN handle RAM?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I've got a simple question. How does XEN handle the RAM?

Some RAM is reserved by Xen itself.

Further ram is reserved for each virtual machine.

> I have a 1 GB machine for instance and want to run 4 domU's, every domU
> will get 256 MB RAM. Is it now still possible to add antoher domU with
> 256 MB RAM? Or is it possible that every domU shares the whole 1 GB of
> RAM, so that all domU's can use the whole RAM, if it is possible.

If you creat an xMB virtual machine, it'll reserve xMB of physical RAM.  Xen 
doesn't swap guests so you can't overcommit memory.  The available memory for 
new guests is:

Total memory - Xen reservation - dom0 memory - total memory for domUs

(things get a bit more complicated for some larger memory systems when not 
using PAE since it's not always possible to address all the memory using 32 
bit physical addresses.  This should not be an issue in your case, however.)

You can't set all the domUs to be able to "burst" up to the whole RAM, but you 
can use the balloon driver to shift memory allocations around between guests 
as needed, if you set them up for it.

Cheers,
Mark

> Any help for that is much appreciated.
>
> kind regards,
>
> andreas

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