KVM¶
KVM is configured as the default hypervisor for Compute.
Note
This document contains several sections about hypervisor selection. If you
are reading this document linearly, you do not want to load the KVM module
before you install nova-compute
. The nova-compute
service depends
on qemu-kvm, which installs /lib/udev/rules.d/45-qemu-kvm.rules
, which
sets the correct permissions on the /dev/kvm
device node.
The KVM hypervisor supports the following virtual machine image formats:
Raw
QEMU Copy-on-write (QCOW2)
QED Qemu Enhanced Disk
VMware virtual machine disk format (vmdk)
This section describes how to enable KVM on your system. For more information, see the following distribution-specific documentation:
Configuration¶
To enable KVM explicitly, add the following configuration options to the
/etc/nova/nova.conf
file:
[DEFAULT]
compute_driver = libvirt.LibvirtDriver
[libvirt]
virt_type = kvm
Enable KVM¶
The following sections outline how to enable KVM based hardware virtualization
on different architectures and platforms. To perform these steps, you must be
logged in as the root
user.
For x86-based systems¶
To determine whether the
svm
orvmx
CPU extensions are present, run this command:# grep -E 'svm|vmx' /proc/cpuinfo
This command generates output if the CPU is capable of hardware-virtualization. Even if output is shown, you might still need to enable virtualization in the system BIOS for full support.
If no output appears, consult your system documentation to ensure that your CPU and motherboard support hardware virtualization. Verify that any relevant hardware virtualization options are enabled in the system BIOS.
The BIOS for each manufacturer is different. If you must enable virtualization in the BIOS, look for an option containing the words
virtualization
,VT
,VMX
, orSVM
.To list the loaded kernel modules and verify that the
kvm
modules are loaded, run this command:# lsmod | grep kvm
If the output includes
kvm_intel
orkvm_amd
, thekvm
hardware virtualization modules are loaded and your kernel meets the module requirements for OpenStack Compute.If the output does not show that the
kvm
module is loaded, run this command to load it:# modprobe -a kvm
Run the command for your CPU. For Intel, run this command:
# modprobe -a kvm-intel
For AMD, run this command:
# modprobe -a kvm-amd
Because a KVM installation can change user group membership, you might need to log in again for changes to take effect.
If the kernel modules do not load automatically, use the procedures listed in these subsections.
If the checks indicate that required hardware virtualization support or kernel modules are disabled or unavailable, you must either enable this support on the system or find a system with this support.
Note
Some systems require that you enable VT support in the system BIOS. If you believe your processor supports hardware acceleration but the previous command did not produce output, reboot your machine, enter the system BIOS, and enable the VT option.
If KVM acceleration is not supported, configure Compute to use a different hypervisor, such as QEMU.
These procedures help you load the kernel modules for Intel-based and AMD-based processors if they do not load automatically during KVM installation.
Intel-based processors
If your compute host is Intel-based, run these commands as root to load the kernel modules:
# modprobe kvm
# modprobe kvm-intel
Add these lines to the /etc/modules
file so that these modules load on
reboot:
kvm
kvm-intel
AMD-based processors
If your compute host is AMD-based, run these commands as root to load the kernel modules:
# modprobe kvm
# modprobe kvm-amd
Add these lines to /etc/modules
file so that these modules load on reboot:
kvm
kvm-amd
For POWER-based systems¶
KVM as a hypervisor is supported on POWER system’s PowerNV platform.
To determine if your POWER platform supports KVM based virtualization run the following command:
# cat /proc/cpuinfo | grep PowerNV
If the previous command generates the following output, then CPU supports KVM based virtualization.
platform: PowerNV
If no output is displayed, then your POWER platform does not support KVM based hardware virtualization.
To list the loaded kernel modules and verify that the
kvm
modules are loaded, run the following command:# lsmod | grep kvm
If the output includes
kvm_hv
, thekvm
hardware virtualization modules are loaded and your kernel meets the module requirements for OpenStack Compute.If the output does not show that the
kvm
module is loaded, run the following command to load it:# modprobe -a kvm
For PowerNV platform, run the following command:
# modprobe -a kvm-hv
Because a KVM installation can change user group membership, you might need to log in again for changes to take effect.
For AArch64-based systems¶
Todo
Populate this section.
Configure Compute backing storage¶
Backing Storage is the storage used to provide the expanded operating system
image, and any ephemeral storage. Inside the virtual machine, this is normally
presented as two virtual hard disks (for example, /dev/vda
and /dev/vdb
respectively). However, inside OpenStack, this can be derived from one of these
methods: lvm
, qcow
, rbd
or flat
, chosen using the
libvirt.images_type
option in nova.conf
on the
compute node.
Note
The option raw
is acceptable but deprecated in favor of flat
. The
Flat back end uses either raw or QCOW2 storage. It never uses a backing
store, so when using QCOW2 it copies an image rather than creating an
overlay. By default, it creates raw files but will use QCOW2 when creating a
disk from a QCOW2 if force_raw_images
is not set in
configuration.
QCOW is the default backing store. It uses a copy-on-write philosophy to delay allocation of storage until it is actually needed. This means that the space required for the backing of an image can be significantly less on the real disk than what seems available in the virtual machine operating system.
Flat creates files without any sort of file formatting, effectively creating files with the plain binary one would normally see on a real disk. This can increase performance, but means that the entire size of the virtual disk is reserved on the physical disk.
Local LVM volumes can also be
used. Set the libvirt.images_volume_group
configuration
option to the name of the LVM group you have created.
Direct download of images from Ceph¶
When the Glance image service is set up with the Ceph backend and Nova is using
a local ephemeral store ([libvirt]/images_type!=rbd
), it is possible to
configure Nova to download images directly into the local compute image cache.
With the following configuration, images are downloaded using the RBD export command instead of using the Glance HTTP API. In some situations, especially for very large images, this could be substantially faster and can improve the boot times of instances.
On the Glance API node in glance-api.conf
:
[DEFAULT]
show_image_direct_url=true
On the Nova compute node in nova.conf:
[glance]
enable_rbd_download=true
rbd_user=glance
rbd_pool=images
rbd_ceph_conf=/etc/ceph/ceph.conf
rbd_connect_timeout=5
Nested guest support¶
You may choose to enable support for nested guests — that is, allow
your Nova instances to themselves run hardware-accelerated virtual
machines with KVM. Doing so requires a module parameter on
your KVM kernel module, and corresponding nova.conf
settings.
Host configuration¶
To enable nested KVM guests, your compute node must load the
kvm_intel
or kvm_amd
module with nested=1
. You can enable
the nested
parameter permanently, by creating a file named
/etc/modprobe.d/kvm.conf
and populating it with the following
content:
options kvm_intel nested=1
options kvm_amd nested=1
A reboot may be required for the change to become effective.
Nova configuration¶
To support nested guests, you must set your
libvirt.cpu_mode
configuration to one of the following
options:
- Host passthrough (
host-passthrough
) In this mode, nested virtualization is automatically enabled once the KVM kernel module is loaded with nesting support.
[libvirt] cpu_mode = host-passthrough
However, do consider the other implications that host passthrough mode has on compute functionality.
- Host model (
host-model
) In this mode, nested virtualization is automatically enabled once the KVM kernel module is loaded with nesting support, if the matching CPU model exposes the
vmx
feature flag to guests by default (you can verify this withvirsh capabilities
on your compute node). If your CPU model does not pass in thevmx
flag, you can force it withlibvirt.cpu_model_extra_flags
:[libvirt] cpu_mode = host-model cpu_model_extra_flags = vmx
Again, consider the other implications that apply to the host model mode.
- Custom (
custom
) In custom mode, the same considerations apply as in host-model mode, but you may additionally want to ensure that libvirt passes not only the
vmx
, but also thepcid
flag to its guests:[libvirt] cpu_mode = custom cpu_models = IvyBridge cpu_model_extra_flags = vmx,pcid
More information on CPU models can be found in CPU models.
Limitations¶
When enabling nested guests, you should be aware of (and inform your users about) certain limitations that are currently inherent to nested KVM virtualization. Most importantly, guests using nested virtualization will, while nested guests are running,
fail to complete live migration;
fail to resume from suspend.
See the KVM documentation for more information on these limitations.
KVM performance tweaks¶
The VHostNet kernel module improves network performance. To load the kernel module, run the following command as root:
# modprobe vhost_net
Troubleshooting¶
Trying to launch a new virtual machine instance fails with the ERROR
state,
and the following error appears in the /var/log/nova/nova-compute.log
file:
libvirtError: internal error no supported architecture for os type 'hvm'
This message indicates that the KVM kernel modules were not loaded.
If you cannot start VMs after installation without rebooting, the permissions
might not be set correctly. This can happen if you load the KVM module before
you install nova-compute
. To check whether the group is set to kvm
,
run:
# ls -l /dev/kvm
If it is not set to kvm
, run:
# udevadm trigger