Memory hot plug: increase the number of slots from '32' to '256' (Documentation)

[bugproxy]

IBM would like to have the following documented wherever appropriate:

---Problem Description---
In Ubuntu 17.04, when trying to increase the number of memory slots from 32 to 256, only 63 slots were available even though 256 slots was specified in the guest xml.

Machine Type = POWER8

Even though 256 slots are supported in QEMU 2.8, distributed with Ubuntu 17.04, there is an implicit limit coming from the vhost backend, limiting the number of max memory slots that are supported.

The work around is to increase the vhost mem slots by reloading the vhost module in the guest:

[root@localhost ~]# modprobe -r vhost
[root@localhost ~]# modprobe vhost max_mem_regions=512
[root@localhost ~]# cat /sys/module/vhost/parameters/max_mem_regions
512

[Gunnar Hjalmarsson (gunnarhj)]

Hi!

You filed this bug report against the ubuntu-docs package, which is a basic user documentation for the Ubuntu desktop. To be honest, what you talk about is far above my head, but I doubt that the topic is suitable for ubuntu-docs.

Can you please provide some further information, so we can figure out which kind of docs (if any) might be affected.

[bugproxy (bugproxy)]

------- Comment From <email address hidden> 2017-02-20 20:12 EDT-------
This is related to KVM guests memory configuration on IBM Power systems.

In the past we documented stuff like this in a wiki [0]. Not sure if Canonical has a more appropriate place to document these restrictions/workarounds.

0. https://wiki.ubuntu.com/ppc64el/uKVM

[Gunnar Hjalmarsson (gunnarhj)]

Well, there is this section in the official server guide:

https://help.ubuntu.com/lts/serverguide/virtualization.html

I changed this report to the server guide project, so the maintainers of that guide can consider your suggestion. Thanks!

[Doug Smythies (dsmythies)]

In the Ubuntu Serverguide we do not go into that level of detail. The virtualization chapter is rather basic. I'm going to set this to "won't fix".

[bugproxy (bugproxy)]

------- Comment From <email address hidden> 2017-06-21 09:44 EDT-------
Canonical,

Any updates on this pending documentation?

[bugproxy (bugproxy)]

------- Comment From <email address hidden> 2017-06-21 10:33 EDT-------
Ok. I ended documenting this one at https://wiki.ubuntu.com/ppc64el/uKVM .