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[Xen-users] Changing Xen hard memory limit in running domain


I have a machine with 1G RAM, running Xen 3.0.1, and Linux.

I want to create two domU VMs and move ALL the memory, flexibly, back and
forth between the two VMs.

To do this, I created two domU VMs running Linux, both with kernel boot
parameters mem=1G, to tell Linux to prepare memory tables for 1G RAM. I booted the VMs, with xm parameter 'memory=256', i.e. telling Xen to only give them
256Mb RAM during the startup.

I wanted to later be able to give 700M RAM to one machine and 300M to the
other, by using the balloon driver ('xm mem-set' command)

Creating and booting the VMs worked fine.

But then I tried to use 'xm mem-set' to give more memory to the VMs, and that did *not* work. It's because Xen has set the 'Xen hard memory limit' (visible
in the VMs' /proc/xen/balloon) to 256M.

I tried to use 'xm mem-max' to change that up to 1G, but 'mem-max' does not
seem to do anything. Is that correct?  Can it be fixed?

There is a bugzilla entry asking for 'mem-max' to be removed, but it would be nice to have a way to boot up a domain with less memory than it is allowed to
have in the future.

Is what I'm trying to do perhaps doable in some way, without using mem-max?

One way that I thought could be an alternative way is this:

I tried to create the VMs one by one, each with 1G of RAM, and ballooning them with mem-set down to 256M before saving them. Then I wanted to restore them, as they then would have the mem-max paramters and Linux memory tables set up

Sadly, they couldn't be restored, because Xen tries to allocate all the memory (= the memory given at the first boot) when it restores the VMs. So only the
first VM could be restored, even though they only used 256M each.

I mentioned this about a year ago, and got the impression that it was being
worked on.  Was it ever fixed?

"out-of-memory problem when restoring ballooned domains"

Lars Rasmusson

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