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Re: [Xen-devel] memory hotplug for domUs



On 01/20/2017 09:06 AM, Juergen Gross wrote:
On 20/01/17 16:54, Ian Jackson wrote:
Juergen Gross writes ("memory hotplug for domUs"):
We first thought to enhance "xl mem-set", but after some more thinking
about it I'd rather add a new xl command, e.g. "mem-add" (we could later
even add "mem-remove" to support memory unplug).

Why ?  Why would xl mem-set not automatically do the right thing ?

How would you specify the numa node to add the memory to?

And the host numa node providing the memory?

Especially for removal of memory ballooning and hotplug are _very_
different.

I agree. libvirt already models ballooning and hot(un)plug differently.

For ballooning, typical VM configuration is

  <memory unit='KiB'>16777216</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <memballoon model='virtio|xen'>

'virsh setmaxmem' and 'virsh setmem' allow adjusting runtime and/or persistent configuration.

For hot(un)plug (currently only supported by the qemu driver) typical configuration is

  <maxMemory slots='16' unit='KiB'>16777216</maxMemory>
  <devices>
    <memory model='dimm'>
      <source>
        <nodemask>0-1</nodemask>
      </source>
      <target>
        <size unit='KiB'>1048576</size>
        <node>0</node>
      </target>
    </memory>
    ...
</devices>

which describes a VM with 16 dimm slots with total capacity of 16G. One slot is filled with a 1G memory module whose memory must come from host numa nodes 0 or 1, and is provided to numa node 0 in the VM.

'virsh attach-device' and virsh detach-device' allow adding or removing memory modules from the dimms. AIUI, under the covers qemu uses ACPI memory hotplug.


This problem with lack of support (and indeed requirementse to do
hotplug things) is a detail that in the long term should not be a
problem, and in the meantime can be dealt with by a force option.

I think it would be clearer with new commands for hotplugging. I
don't feel very strong about this, so in case everyone else is fine
with handling everything via mem-set I won't object.

If ACPI memory hotplug is indeed the goal, then I agree new commands should be used since it is quite different than ballooning.

Regards,
Jim


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