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kvm.md

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KVM/libvirt

Shrink qcow2 files

  1. Delete any extraneous files, like package archives, cache, etc.
  2. Zero out the virtual disk:
    • Use BleachBit with the 'Free disk space' option checked, or
    • Pipe from /dev/zero to a file. (Be sure to delete it after!)
  3. Shut down the VM and pass the qcow2 file to the shrink-vm script.

Enable virgl 3D rendering in Virtual Machine Manager

  1. Switch the Video mode from 'QXL' to 'Virtio,' and be sure to check the 3D acceleration option.
  2. Switch the SPICE display server listen type to 'None', and check the OpenGL box.
  3. Enable cgroup_device_acl in /etc/livbirt/qemu.conf and add '/dev/dri/renderD128' to the list. Restart libvirtd.service afterwards.

Delete image snapshots

To view available snapshots for a particular domain, run:

virsh -c qemu:///system snapshot-list --domain <DOMAIN_ID>

Delete any of the listed images by:

virsh -c qemu:///system snapshot-delete --domain <DOMAIN_ID> <SNAPSHOT_ID>

Delete domains

virsh -c qemu:///system undefine <DOMAIN_ID>

In the event that non-stock emulation of NVRAM, pass the --nvram flag, too. This is relevant for OSX-KVM.

virsh -c qemu:///system undefine --nvram <DOMAIN_ID>

Convert OVA file to QCOW2

To convert a VirtualBox image in OVA format to a QCOW2 image compatible with QEMU, run:

tar xf <input>
qemu-img convert -O qcow2 <input> <output>

Share a directory between host and guest with virtiofs

To share the host directory <hostdir> and mount it within a VM at the directory <guestdir>, do the following:

  1. From the Virt-Manager GUI, select the 'Show virtual hardware details' icon.
  2. In the 'Memory' section, check the 'Enable shared memory' box.
  3. Select 'Add Hardware' at the bottom of the UI.
  4. In the pop-up, select 'Filesystem.'
  5. Select the 'virtiofs' driver (the default).
  6. Enter the absolute path of <hostdir> in the 'Source path' text box.
  7. Enter the target string (e.g. share) in 'Target path.' This value is not the <guestdir> path.
  8. Confirm the changes (i.e. click 'Finish') and boot the VM.
  9. From within the guest, run: 
    sudo mount -t virtiofs <share> <guestdir>
    For clarity, <share> is the target string, and <guestdir> is the absolute path for the mountpoint of the shared directory in the guest.

Additionally, create mount rules in /etc/fstab to avoid having to manually mount the shared filesystem at each boot. Add a rule as follows:

<share>	<guestdir>	virtiofs	defaults	0	0