HON’s Wiki # libvirt & KVM

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Contents

• General
• Installation (Debian)
• Usage
  • General
  • Manage VMs
  • Networking
  • Storage
  • Snapshots
  • Tuning
  • Miscellanea

libvirt is a tool for managing platform virtualization like KVM and QEMU (among others). I’ll only focus on using it with KVM (and QEMU) here.

General

• Requires Intel VT or AMD-V to be enabled to function optimally.
• Note that running commands as non-root or not in the libvirt group will generally work but will not show all VMs. etc.
• KVM was merged into the Linux kernel mainline in kernel version 2.6.20.

Installation (Debian)

1. Install without extra stuff (like GUIs): apt-get install --no-install-recommends iptables bridge-utils qemu-system qemu-utils libvirt-clients libvirt-daemon-system virtinst libosinfo-bin
2. (Optional) Install dnsmasq-base for accessing guests using their hostnames.
3. (Optional) Add users to the libvirt group to allow them to manage libvirt without sudo. Otherwise, remember to always specify use sudo to use the correct context/system URI/whatever.
4. Set up the default network:
   1. It’s already created, using NAT, DNS and DHCP.
   2. If not using dnsmasq, disable DNS and DHCP:
      1. Open the config: virsh net-edit default
      2. Disable DNS: Set <dns enable='no'/>.
      3. Disable DHCP: Remove <dhcp>...</dhcp>.
      4. Restart libvirtd.
   3. Start it: virsh net-start default
   4. (Optional) Autostart it: virsh net-autostart default
5. Set up the default storage pool:
   1. Create it: virsh pool-define-as default --type=dir --target=/var/lib/libvirt/images
   2. Start it: virsh pool-start default
   3. Autostart it: virsh pool-autostart default

Usage

General

• Use LIBVIRT_DEFAULT_URI=qemu:///system to explicitly use the system URI (rarely needed).
• Show version: virsh version

Manage VMs

• Show VMs: virsh list --all
• Show VM details: virsh dominfo <vm>
• Start/shutdown/reboot/kill VM: virsh {start | shutdown | reboot | kill} <vm>
• Suspend/resume VM: virsh {suspend | resume} <vm>
• Enter/exit console for VM:
  • Enter: virsh console <vm>
  • Exit: Ctrl+] (US) or Ctrl+¨ (NO)
• Create VM:
  • Example: virt-install --name=example-vm --os-variant=debian10 --ram=$((1*1024)) --vcpus=1 --disk=path=/var/lib/libvirt/images/example-vm.qcow2,bus=virtio,size=5 --network=network=default,model=virtio --graphics=none --extra-args="console=ttyS0" --location=debian-10.7.0-amd64-netinst.iso
  • The disk path should match a storage pool path.
  • Show available OS variants: osinfo-query os
  • To disable various checks, specify --check=all=off.
  • To use a host bridge instead of a libvirt network, specify --network=bridge=something instead of --network=network=something.
  • To avoid opening a console (or waiting if there is no console), specify --noautoconsole to avoid that.
  • If it “can’t find the kernel” when using --location, use --cdrom instead. This disallows using arguments like --extra-args, so you’ll need to find another console.
  • Make sure the ISO is readable by the QEMU user.
  • To install using a VNC screen instead of console (e.g. if you need richer graphics or when using --cdrom and no console is allocated), replace --graphics=none with --noautoconsole --graphics=vnc,password=<password>. It only binds to localhost by default, so use something like SSH port forwarding (ssh -L 5900:127.0.0.1:5900 <user>@<addr>) to access it remotely. Remember to disable it in the VM config when no longer needed.
• Clone VM:
  1. Create a source/template VM and make sure it’s not running.
  2. Clone it: virt-clone --original=<source-vm> --name=<vm> -f <vm>.qcow2
• Remove VM: virsh undefine <vm>
• Set VM to automatically start (or disable it): virsh autostart [--disable] <vm>
• Edit VM config:
  1. Open tmp. config in editor: virsh edit <vm>
  2. Shutdown and start the VM (rebooting it doesn’t work).
• Show VM config: virsh dumpxml <vm>
• Show VM graphics URI: virsh domdisplay <vm>
  • For VNC, the shown port is offset from port 5900.
• Run QEMU monitor command: qemu-monitor-command <vm> --hmp <command>
• Show VM logs: See log dir /var/log/libvirt/qemu/.

Networking

• The default network interface is virbr0, called default in libvirt.
• Show networks: virsh net-list
• Show network config: virsh net-dumpxml <network>
• Edit network config (without applying it): virsh net-edit <network>
• Apply changed network config: Restart libvirt or reboot the system.
• Create bridge connected to physical NIC:
  • (Note) If you’re connected remotely, try to avoid locking yourself out.
  • Create bridge on the host: See BridgeNetworkConnections (Debian Wiki) or something.

Storage

• Effectively, the pool is a directory while the files in it (typically disk images) are the volumes.
• Pool basics:
  • Show pools: virsh pool-list [--all] [--details]
• Volume basics:
  • Show volumes: virsh list-vol <pool> [--details]
  • Show volume info: virsh vol-info <file>
• Resize disk: qemu-img resize <file> <size-change> (e.g. +1G)
• Cold backup of VM:
  • Make sure the VM is stopped so that the disk image is consistent.
  • Backup the image in /var/lib/libvirt/images/.
  • Backup the config in /etc/libvirt/qemu/.

Snapshots

• The current snapshot means the last one.
• List snapshots for a VM: virsh snapshot-list <vm>
• Show snapshot info: virsh snapshot-info --domain=<vm> [--current]
• Create snapshot: virsh snapshot-create-as --domain=<vm> --name=<snapshot> --description=<description>
• Revert a snapshot:
  1. Stop the VM.
  2. Revert a named or the current snapshot: virsh snapshot-revert {--domain=<vm> | --current} --snapshotname=<snapshot> [--running]

Tuning

• Assign more CPU cores. When adding many, attempt to assign every hyperthreaded twin to the same VM.
• TODO: Cache mode write-through and write-back.
• Consider enabling huge pages.
• Consider using memory ballooning to save memory on the host when the VM doesn’t need it.

Miscellanea

• To repair a corrupted QEMU disk, try using guestfish.

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