Re: [Xen-devel] RAM allocation and miscellaneous questions

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

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

Cool!

> can I allocate more RAM to each VM than arithmetic would suggest? 

Yes and no ;-)

Xen does not do paging to disk, so you can't (for instance) tell a domain it 
has 256MB but only give it 128MB and then fake out the extra capacity using 
disk.  The solution under Xen would be to just get the domain to do its own 
paging - this has the same end result and avoids certain performance and 
implementation nasties associated with paging in the VMM.

Using the balloon driver, you can have a setup where you add and remove memory 
to / from domains according to their need, so that you can shrink the memory 
footprints of domains that don't (currently) need as much in core.  This is 
done manually.

Of course, you can also use suspend / resume and live migration to move 
domains around in order to balance memory load.

> Is there some kind of compression or COW 
> that goes on in the Xen memory management code?

Not right now.  There is a plan to implement a shared buffer cache, which 
would allow domains to share unmodified pages of data that have been read 
from disk.  This is planned as a performance optimisation not a space 
optimisation, however.

> 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?

It probably depends.  I guess you'd save some memory footprint that way 
(compared to running many copies of the servers) but I really don't know how 
much.

> [64 bit machines]
> Does Xen work well with Opterons?

In 32-bit mode, it works.  In 64-bit mode, not quite yet.  x86_64 support is 
under development at the moment and is scheduled as a post-2.0 feature.  
However that will be the preferred path to large memory configurations.

HTH,
Mark


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