17.4. Configuring virtual machine memory

To improve the performance of a virtual machine (VM), you can assign additional host RAM to the VM. Similarly, you can decrease the amount of memory allocated to a VM so the host memory can be allocated to other VMs or tasks.

To perform these actions, you can use the web console or the command-line interface.

17.4.1. Adding and removing virtual machine memory using the web console

To improve the performance of a virtual machine (VM) or to free up the host resources it is using, you can use the web console to adjust amount of memory allocated to the VM.

Conditions préalables

  • The guest OS is running the memory balloon drivers. To verify this is the case:

    1. Ensure the VM’s configuration includes the memballoon device: 

      # virsh dumpxml testguest | grep memballoon
<memballoon model='virtio'>
    </memballoon>

      If this commands displays any output and the model is not set to none, the memballoon device is present.

    2. Ensure the balloon drivers are running in the guest OS.

  • The web console VM plug-in is installed on your system.

Procédure

  1. Optional: Obtain the information about the maximum memory and currently used memory for a VM. This will serve as a baseline for your changes, and also for verification. 

    # virsh dominfo testguest
Max memory:     2097152 KiB
Used memory:    2097152 KiB

  2. In the Virtual Machines interface, click the VM whose information you want to see.

    A new page opens with an Overview section with basic information about the selected VM and a Console section to access the VM’s graphical interface.

  3. Click edit next to the Memory line in the Overview pane.

    The Memory Adjustment dialog appears.

    Image displaying the VM memory adjustment dialog box.

  4. Configure the virtual CPUs for the selected VM.

    • Maximum allocation - Sets the maximum amount of host memory that the VM can use for its processes. You can specify the maximum memory when creating the VM or increase it later. You can specify memory as multiples of MiB or GiB.

      Adjusting maximum memory allocation is only possible on a shut-off VM.

    • Current allocation - Sets the actual amount of memory allocated to the VM. This value can be less than the Maximum allocation but cannot exceed it. You can adjust the value to regulate the memory available to the VM for its processes. You can specify memory as multiples of MiB or GiB.

      If you do not specify this value, the default allocation is the Maximum allocation value.

  5. Cliquez sur Enregistrer.

    The memory allocation of the VM is adjusted.

Ressources supplémentaires

17.4.2. Adding and removing virtual machine memory using the command-line interface

To improve the performance of a virtual machine (VM) or to free up the host resources it is using, you can use the CLI to adjust amount of memory allocated to the VM.

Conditions préalables

  • The guest OS is running the memory balloon drivers. To verify this is the case:

    1. Ensure the VM’s configuration includes the memballoon device: 

      # virsh dumpxml testguest | grep memballoon
<memballoon model='virtio'>
    </memballoon>

      If this commands displays any output and the model is not set to none, the memballoon device is present.

    2. Ensure the ballon drivers are running in the guest OS.

Procédure

  1. Optional: Obtain the information about the maximum memory and currently used memory for a VM. This will serve as a baseline for your changes, and also for verification. 

    # virsh dominfo testguest
Max memory:     2097152 KiB
Used memory:    2097152 KiB

  2. Adjust the maximum memory allocated to a VM. Increasing this value improves the performance potential of the VM, and reducing the value lowers the performance footprint the VM has on your host. Note that this change can only be performed on a shut-off VM, so adjusting a running VM requires a reboot to take effect.

    For example, to change the maximum memory that the testguest VM can use to 4096 MiB: 

    # virt-xml testguest --edit --memory memory=4096,currentMemory=4096
Domain 'testguest' defined successfully.
Changes will take effect after the domain is fully powered off.

    To increase the maximum memory of a running VM, you can attach a memory device to the VM. This is also referred to as memory hot plug. For details, see Attaching memory devices to virtual machines.

    Avertissement

    Removing memory devices from a running VM (also referred as a memory hot unplug) is not supported, and highly discouraged by Red Hat.

  3. Optional: You can also adjust the memory currently used by the VM, up to the maximum allocation. This regulates the memory load that the VM has on the host until the next reboot, without changing the maximum VM allocation. 

    # virsh setmem testguest --current 2048

Vérification

  1. Confirm that the memory used by the VM has been updated: 

    # virsh dominfo testguest
Max memory:     4194304 KiB
Used memory:    2097152 KiB

  2. Optional: If you adjusted the current VM memory, you can obtain the memory balloon statistics of the VM to evaluate how effectively it regulates its memory use. 

     # virsh domstats --balloon testguest
Domain: 'testguest'
  balloon.current=365624
  balloon.maximum=4194304
  balloon.swap_in=0
  balloon.swap_out=0
  balloon.major_fault=306
  balloon.minor_fault=156117
  balloon.unused=3834448
  balloon.available=4035008
  balloon.usable=3746340
  balloon.last-update=1587971682
  balloon.disk_caches=75444
  balloon.hugetlb_pgalloc=0
  balloon.hugetlb_pgfail=0
  balloon.rss=1005456

Ressources supplémentaires

17.4.3. Ressources supplémentaires