HON95-wiki/config/virt-cont/libvirt-kvm.md

---

title: libvirt & KVM breadcrumbs:

• title: Configuration
• title: Virtualization & Containerization --- {% include header.md %}

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.

Using Debian.

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

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.
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.

{% include footer.md %}