Re: [PATCH] balloon: selfballooning and post memory info via xenbus,

[Chris Lalancette <clalance@redhat.com>]

Dan Magenheimer wrote:
>>> memory = 256
>>> maxmem = 8192
> 
> By the way, I'm not sure if you knew this, but the above two
> lines don't work as you might want.  The maxmem is ignored.
> The domain is launched (in this example) with 256MB of
> memory and (at least without hot-plug memory support in the
> guest) memory can only be decreased from there, not increased.

Assuming we are talking about PV guests, I think this is wrong.  My knowledge is
a little dated (mostly 3.1.x series knowledge), but unless it has changed, this
should work.  As I understand it, what happens is that if you specify like
above, dom0 gets ballooned down 256MB, and then your domain is started with
256MB.  From there, you should be able to use xm mem-set <domid> <MB> to set the
amount of memory in the guest, up to maxmem.

But there is a big caveat, which trips people up all of the time.  The xm
mem-set command will *not* automatically balloon down dom0 for you.  So if you
allocated all memory to dom0 on bootup (say, 4GB), then started just this one
guest (so now dom0 == 3.75GB, domU == 256MB), and try to xm mem-set, you will
fail.  If you then xm mem-set 0 3000 (or something), you would then be able to
balloon up the domU an additional .75GB.

Chris Lalancette