Hotplug (qemu disk,nic,cpu,memory)

Proxmox support hotplugging of Qemu devices:

- Disks (hotplug/unplug)

- NICs (hotplug/unplug)

- CPU (hotplug/unplug)

- Memory (hotplug/unplug)

Linux Guests Notes

Kernel Modules Needed

You need to have 2 modules loaded in your Linux guest:

#modprobe acpiphp
#modprobe pci_hotplug

CPU and Memory Hotplug

You need a recent kernel (> 3.10) and a udev rules file, to auto-enable CPU or memory hotplugging.

You can add the file:

/lib/udev/rules.d/80-hotplug-cpu-mem.rules

SUBSYSTEM=="cpu", ACTION=="add", TEST=="online", ATTR{online}=="0", ATTR{online}="1"
SUBSYSTEM=="memory", ACTION=="add", TEST=="state", ATTR{state}=="offline", ATTR{state}="online"

For Linux kernel >= 4.7, you don't need the udev rule for memory hotplug; you only need to add this kernel parameter at boot:

memhp_default_state=online

For Linux kernel >=5.15 (e.g. Ubuntu 22.04) you only need the UDEV rule for cpu hotplug:

/lib/udev/rules.d/80-hotplug-cpu.rules

SUBSYSTEM=="cpu", ACTION=="add", TEST=="online", ATTR{online}=="0", ATTR{online}="1"

Windows Guests Notes

- Disk and NIC hotplug work out of the box for all Windows versions >2003

- CPU and memory hotplug work on > 2008 enterprise/datacenter, 2012 > standard

Disk and NIC Hotplug/unplug

Edit your VM configuration file:

/etc/pve/qemuserver/<VMID>.conf

and add:

hotplug: network,disk,....

Note: Since Proxmox 3.3, the hotplug option is available in the GUI.

Then add/remove your disks or NICs as usual.

For disk unplug, verify that your disk is not mounted in your Linux guest or enabled in Windows Disk Management.

CPU Hotplug

1. In the VM Options, edit the "Hotplug" Value and activate "CPU Hotplug".
2. In the Hardware Section of your VM, specify the maximum number of Sockets/Cores.
3. You can now use the "CPU Options" dialog to change the number of vCPUs, while the machine is running.

maxcpus = sockets * cores.

Note : Windows CPU hotplug has been working since qemu 2.6 and CPU hot unplug has been working since qemu 2.7.

Memory Hotplug

Memory hotplug is available in proxmox 3.4.

You need to enable NUMA and hotplug to get it working:

numa: 1
hotplug: memory,...

Then you can use the memory GUI options as usual.

Memory Hot Unplug

- Memory unplug currently does not work on Windows (<=win10 currently)

- Memory unplug can be unstable on Linux (<= kernel 4.10 currently)

For Linux memory unplug, you need to have movable zone enabled, in the kernel config (not enabled by default on Debian/Ubuntu):

CONFIG_MOVABLE_NODE=YES

and the "movable_node" boot kernel parameter enabled.

Here are some future kernel patches to improve unplug : http://www.spinics.net/lists/linux-mm/msg125359.html

Update max_mem_regions when hotplug is for more than 64GB

For Error : a used vhost backend has no free memory slots left 

Kernel sets how many memory dimms per vhost it supports, default 64. You can increase the limit by creating a file /etc/modprobe.d/vhost.conf

Containing options vhost max_mem_regions=128 (default is 64, you can try increasing it further if necessary) and then rebooting the Proxmox machine.

echo "options vhost max_mem_regions=128" > /etc/modprobe.d/vhost.conf
rmmod vhost_net
rmmod vhost
modprobe vhost_net
cat /sys/module/vhost/parameters/max_mem_regions
# 128

Ref Forum link : https://forum.proxmox.com/threads/problem-with-hotplug-and-64gb-ram.73621/page-3#post-600595