When working with KVM (Kernel-based Virtual Machine), administrators often need to adjust resource allocation for running VMs. Unlike some other hypervisors, KVM allows dynamic modification of both memory and CPU resources without requiring a full VM restart in many cases.

Before proceeding, ensure:

1. Your KVM/libvirt environment is properly configured

2. The VM uses QEMU/KVM virtualization

3. You have appropriate permissions (typically root or via sudo)

4. The host has sufficient available resources

For active VMs using libvirt, the virsh command-line tool provides direct control. To increase vCPUs:

virsh setvcpus --domain vm_name --count 4 --live

To make the change persistent across reboots:

virsh setvcpus --domain vm_name --count 4 --config

Memory adjustment requires slightly different handling. For a running VM:

virsh setmem vm_name 8192 --live

To configure the permanent memory setting:

virsh setmaxmem vm_name 16384 --config
virsh setmem vm_name 8192 --config

Note: The "setmaxmem" command defines the upper limit before setting the current allocation.

For more complex changes, directly edit the VM's XML configuration:

virsh edit vm_name

Locate and modify these sections:

<memory unit='KiB'>8388608</memory>
<currentMemory unit='KiB'>4194304</currentMemory>
<vcpu placement='static'>4</vcpu>

- Some guest OSes require proper acpi configuration for hot-plugging CPUs

- Memory balloon driver must be installed in the guest for dynamic memory

- NUMA configurations may require additional parameters

- Always monitor qemu process memory usage after changes

For batch operations, combine virsh commands with shell scripts:

#!/bin/bash
VM_LIST="web1 db2 app3"
for vm in $VM_LIST; do
    virsh setvcpus $vm 2 --live
    virsh setmem $vm 4096 --live
done

If changes don't take effect:

- Verify guest agent is running (for memory ballooning)

- Check dmesg and /var/log/libvirt/qemu logs

- Confirm the hypervisor supports the requested configuration

- Some older VM definitions may need conversion to newer XML formats

Before modifying resources, ensure:

• The VM is shut down (for certain changes)
• Libvirt tools are installed (sudo apt-get install libvirt-clients)
• You have sudo privileges
• Host has sufficient available resources

# View current configuration
virsh dumpxml vm_name | grep -E "memory|vcpu"

# Edit VM configuration
virsh edit vm_name

Locate these sections in the XML:

<memory unit='KiB'>2097152</memory>  <!-- 2GB RAM -->
<currentMemory unit='KiB'>2097152</currentMemory>
<vcpu placement='static'>2</vcpu>  <!-- 2 CPU cores -->

# Hot-add CPUs (requires guest OS support)
virsh setvcpu vm_name 4 --live --config

# Verify changes
virsh vcpucount vm_name

# Set memory to 4GB (4096MB)
virsh setmem vm_name 4096 --live --config

# Check new allocation
virsh dommemstat vm_name

• For Windows VMs, install virtio drivers before hot-plug
• Some guest OSes require manual CPU/RAM recognition
• Maximum memory cannot exceed original allocation without shutdown
• Use virsh dominfo vm_name to verify changes

#!/bin/bash
VM_NAME="ubuntu-server"
NEW_CPUS=4
NEW_MEM_GB=8

virsh setvcpu "$VM_NAME" "$NEW_CPUS" --config
virsh setmem "$VM_NAME" $((NEW_MEM_GB*1024)) --config
echo "Resources updated: $NEW_CPUS vCPUs, ${NEW_MEM_GB}GB RAM"