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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 22/5/08 08:15, "Chris Lalancette" <clalance@xxxxxxxxxx> wrote:

>> 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.

This is certainly how it is supposed to still work!

 -- Keir



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