RE: Re: GSoC 2010 - Migration from memory ballooning to memory hotplug in Xen

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Andi Kleen [mailto:andi@firstfloor.org]
> 
> Daniel Kiper <dkiper@net-space.pl> writes:
> >
> > OK, let's go to details. When I was playing with Xen I saw that
> > ballooning does not give possibility to extend memory over boundary
> > declared at the start of system. Yes, I know that is by desing
> however
> > I thought that it is a limitation which could by very annoing in some
> > enviroments (I think especially about servers). That is why I decided
> to
> > develop some code which remove that one. At the beggining I thought
> > that it should be replaced by memory hotplyg however after some test
> > and discussion with Jeremy we decided to link balooning (for memory
> > removal) with memory hotplug (for extending memory above boundary
> > declared at the startup of system). Additionaly, we decided to
> implement
> > this solution for Linux Xen gustes in all forms (PV/i386,x86_64 and
> > HVM/i386,x86_64).
> 
> While you can do that the value is not very large because you
> could just start the guests with more memory, but ballooned in
> the first place (so that they don't actually use it)
> 
> The only advantage of using memory hotadd is that the mem_map doesn't
> need to be pre-allocated, but that's only a few percent of the memory.
> 
> So it would only help if you want to add gigantic amounts of memory
> to a VM (like >20-30x of what it already has).

One can envision a scenario where a cloud customer launches a
business-critical VM with some reasonably large "maxmem" set,
balloons up to the max, then finds out it isn't enough after
all and would like to avoid rebooting.  Or a cloud provider
might charge for a specific maxmem, but allow the customer
to increase maxmem if they pay more money.

Dan