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

[Paul Dorman <pauld@xxxxxxxxx>]

It certainly does Mark (help, that is). I would like to learn more about this 
balloon driver, which could make things very managable. Is there any 
documentation or tutorial-like info about?

Also, though my domains are running happily, I am seeing plenty of 
memory-related errors. I've just hooked up a serial cable between my 
development servers and my desktop (I live with fans) so I'll be able to 
furnish the list with more details.

I have been wondering why my Domain-0 machine hasn't been using swap! :o) So I 
should just dump that partition? I suppose I'll have to be very rigorous when 
it comes to keeping that domain lean yes? (Especially as I only have 4GB to 
play with!). Out side of basic OS functionality, are there any Xen or LVM2 
related processes I have to account for when setting the amount of RAM I can 
use for Domain-0?

I don't suppose I could use the ballon driver for Domain-0 too!?!

Thanks for your reply.

Regards,
Paul

On Wednesday 29 September 2004 03:30 pm, Mark A. Williamson wrote:
> > 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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