Growing a para-virtualized domain beyond its initial allocation?

Growing a para-virtualized domain beyond its initial allocation?

[John Byrne]

I didn't get any responses on xen-users, so I'll try xen-devel:

This is somewhere between a user and a developer question, depending on
the answers. My tests were on the latest SuSE drop of xen: 3.0.2_09434-0
on i386.

I want to start a domain with a small amount of memory, but have the
potential to give it more if it needs it. (Perhaps after migrating to a
machine with more memory available.)

A simple "xm mem-set" doesn't work like I want, Linux never sees the
memory. Having looked at the balloon driver code, it would seem that
specifying "mem=256M" on the kernel command line and "memory = 128M" and
"maxmem = 256M" in the config file should do the right things, but in
practice it doesn't work. When I try the "xm mem-set 4 256",
/proc/xen/balloon in the guest reports that I have hit the Xen hard
limit. (I haven't yet figured out where this is coming from.)

So, am I on the right track, but there is a bug? Should I be going about
this another way? Is it hopeless? Assuming this works for
para-virtualized domains, is there any hope for doing the same with a
hvm-domain. (I'd guess not, but I'm supposed to ask.)

Thanks,

John Byrne

Re: Growing a para-virtualized domain beyond its initial allocation?

[Anthony Liguori]

John Byrne wrote:
>
>
> I didn't get any responses on xen-users, so I'll try xen-devel:
>
> This is somewhere between a user and a developer question, depending on
> the answers. My tests were on the latest SuSE drop of xen: 3.0.2_09434-0
> on i386.
>
> I want to start a domain with a small amount of memory, but have the
> potential to give it more if it needs it. (Perhaps after migrating to a
> machine with more memory available.)
>
> A simple "xm mem-set" doesn't work like I want, Linux never sees the
> memory. Having looked at the balloon driver code, it would seem that
> specifying "mem=256M" on the kernel command line and "memory = 128M" and
> "maxmem = 256M" in the config file should do the right things, but in
> practice it doesn't work. When I try the "xm mem-set 4 256",
> /proc/xen/balloon in the guest reports that I have hit the Xen hard
> limit. (I haven't yet figured out where this is coming from.)

You ought to be able to set:

memory=128
maxmem=256

In a domain configuration file.  I was poking around with the maxmem 
option the other day and noticed that it wasn't being transmitted in xm 
create (or it at least appeared that way).  I was in the process of 
hunting down another bug so I didn't look too much into it.  If the 
above doesn't give you your desired behavior, you should open a bug in 
bugzilla about it.

Regards,

Anthony Liguori

> So, am I on the right track, but there is a bug? Should I be going about
> this another way? Is it hopeless? Assuming this works for
> para-virtualized domains, is there any hope for doing the same with a
> hvm-domain. (I'd guess not, but I'm supposed to ask.)
>
> Thanks,
>
> John Byrne
>
>
>
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