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This is a general place to put hints and tips for libvirt.

Debian/Ubuntu guests under KVM don't shut down properly

KVM just sends an ACPI signal to the guest to tell it to shut down. Of course, this means the guest needs to do something useful when it receives the signal! By default Debian/Ubuntu guests don't.

Solution: install acpid in the guest.

(Thanks to Soren Hansen)

Libvirt wont connect to qemu hypervisor (KVM)

Clean install on Ubuntu 8.10 desktop. Built KVM from scratch, and installed in default location. Built LibVirtD from scratch, and also installed in the default location. Entering virsh -c qemu:///system gave an error about not being able to connect to the hypervisor.

The problem is the hard-coded list of places it looks for qemu/kvm:

if ((virFileExists("/usr/bin/qemu")) ||
      (virFileExists("/usr/bin/qemu-kvm")) ||
      (virFileExists("/usr/bin/kvm")) ||
      return 1;

The solution was to symlink the real qemu launcher program to one of the above locations.

UPDATE: As of the 0.7.6 release, libvirt will now search in $PATH for all the binaries

The connection fails using md5 digest auth

virsh will just say:

failed to connect to the hypervisor

and virt-manager:

Failed to start SASL negotiation: -4 (SASL(-4): no mechanism available: No worthy mechs found)

Make sure all the necessary SASL libraries are installed. On Debian/Ubuntu the package libsasl2-modules is necessary to get it to work.

Enabling debug output for libvirtd and virsh

export  LIBVIRT_DEBUG=yes

Will enable debug messages. Provided ENABLE_DEBUG is specified at the ./configure stage (i think). This is very helpful in diagnosing problems.

Increasing the disk size of a virtual machine

Consider the case of a VM created with default storage size, where the guest is running Fedora on an lvm partition. If the VM runs out of disk space, because the default disk allocation turned out to be insufficient, then it is desirable to allocate more storage from the host. By far the easiest way to do this is to use virt-resize.

However, using virt-resize requires storage space on the host for both the old and new images at the same time.

Another method is to use virt-rescue which gives you a text-based rescue CD containing parted. This lets you move and manipulate partitions in the guest by hand (be careful).

A third way is as follows (example using a Fedora 12 host):

  • Increase the size of the backing file on the host, then inform libvirtd of the change:
cd /var/libvirt/images
truncate --size=+2G storage.img
virsh pool-refresh default

default may vary if you use other storage options. Use the following command to list all available pools:

virsh pool-list --all
  • Update the VM description to boot from a LiveDVD image that can access partition resizing tools (for example, the latest Fedora image The virt-manager gui makes this easier:
choose the VM to be resized
gracefully shut it down
Edit->Virtual Machine Details
select Boot Options, set Boot Device to CDROM, and Apply
select IDE CDROM 1, Connect, and browse to .iso image
  • Boot the VM using the LiveDVD image, to rearrange partitions. Once booted, open a terminal, and:
yum install gparted system-config-lvm
 - select the free space
 - create a new ext4 partition
 - apply changes
 - exit
 - under Uninitialized Entities, find the newly-created partition, and select Initialize
 - select add to an existing volume group, and choose the correct group name
 - under Volume Groups, find the Logical View of the modified volume group, and Edit Properties
 - change the size of the logical volume to consume the free space remaining from the volume group
  • Update the VM description back to normal disk boot. The virt-manager gui makes this easier:
choose the VM that was just resized
Edit->Virtual Machine Details
select Boot Options, set Boot Device to Hard Disk, and Apply
select IDE CDROM 1, Disconnect
  • Boot the VM, and check that the disk size increased as desired:

Adding an alternative storage pool

If the default storage location of /var/lib/libvirt/images does not have enough space for the volumes that you wish to assign to virtual machines, you can add a second storage pool. These steps may prove helpful:

virsh pool-dumpxml default > pool.xml
edit pool.xml # with new name and path
virsh pool-create pool.xml
virsh pool-refresh name

One side note: You will need to delete the UUID field in the xml file if you intend to keep the default pool - no two pools defined can have the same UUID - but keep the brackets like so: '<uuid></uuid>'