Fedora Infrastructure Libvirt Notes

Notes/FAQ on using libvirt/virsh/virt-manager in our environment

How do I migrate a guest from one virthost to another

  1. Setup an unpassworded root ssh key to allow communication between the two virthosts as root. This is only temporary, so, while scary it is not a big deal. Right now, this also means modifying the /etc/ssh/sshd_config to permitroot without-password.

  2. Determine whatever changes need to be made to the guest. This can be the number of cpus, the amount of memory, or the disk location as this may not be standard on the current server.

  3. Make a dump of the current virtual guest using the virsh command. Use virsh dumpxml --migratable guestname and then edit any changes in disk layout, memory and cpu needed.

  4. setup storage on the destination end to match the source storage. lvs will give the amount of disk space. Due to some vaguries on disk sizes, it is always better to round up so if the original server says it is using 19.85 GB, make the next image 20 GB. On the new server, use lvcreate -L+${SIZE}GB -n ${FQDN} vg_guests

  5. as root on source location:

    virsh -c qemu:///system  migrate --xml ${XML_FILE_FROM_3} \
  --copy-storage-all ${GUESTNAME} \
  qemu+ssh://root@destinationvirthost/system

This should start the migration process and it will output absolutely
jack-squat on the cli for you to know this. On the destination system
go look in /var/log/libvirt/qemu/myguest.log (tail -f will show you the
progress results as a percentage completed)

  6. Once the migration is complete you will probably need to run this on the new virthost:

    scp ${XML_FILE_FROM_3} root@destinationvirthost:

ssh root@destinationvirthost
virsh define ${XML_FILE_FROM_3}
virsh autostart ${GUESTNAME}

  7. Edit ansible host_vars of the guest and make sure that the associated values are correct:

    volgroup: /dev/vg_guests
vmhost: virthost??.phx2.fedoraproject.org

  8. Run the noc.yml ansible playbook to update nagios.

This should work for most systems. However in some cases, the virtual servers on either side may have too much activity to 'settle' down enough for a migration to work. In other cases the guest may be on a disk like ISCSI which may not allow for direct migration. In this case you will need to use a more direct movement.

  1. Schedule outage time if any. This will need to be long enough to copy the data from one host to another, so will depend on guest disk size.

  2. Turn off monitoring in nagios

  3. setup an unpassworded root ssh key to allow communication between the two virthosts as root. This is only temporary, so, while scary it is not a big deal. Right now, this also means modifying the /etc/ssh/sshd_config to permitroot without-password.

  4. Determine whatever changes need to be made to the guest. This can be the number of cpus, the amount of memory, or the disk location as this may not be standard on the current server.

  5. Make a dump of the current virtual guest using the virsh command. Use virsh dumpxml --migratable guestname and then edit any changes in disk layout, memory and cpu needed.

  6. setup storage on the destination end to match the source storage. lvs will give the amount of disk space. Due to some vaguries on disk sizes, it is always better to round up so if the original server says it is using 19.85 GB, make the next image 20 GB. On the new server, use lvcreate -L+${SIZE}GB -n ${FQDN} vg_guests

  7. Shutdown the guest.

  8. Insert iptables rule for nc transfer:

    iptables -I INPUT 14 -s <source host> -m tcp -p tcp --dport 11111 -j ACCEPT

  9. On the destination host:

    • RHEL-7:

      nc -l 11111 | dd of=/dev/<guest_vg>/<guest-partition>

  10. On the source host:

    dd if=/dev/<guest_vg>/<guest-partition> | nc desthost 11111

    Wait for the copy to finish. You can do the following to track how far something has gone by finding the dd pid and then sending a 'kill -USR1' to it.

  11. Once the migration is complete you will probably need to run this on the new virthost:

    scp ${XML_FILE_FROM_3} root@destinationvirthost:

ssh root@destinationvirthost
virsh define ${XML_FILE_FROM_3}
virsh autostart ${GUESTNAME}

  12. Edit ansible host_vars of the guest and make sure that the associated values are correct:

    volgroup: /dev/vg_guests
vmhost: virthost??.iad2.fedoraproject.org

  13. Run the noc.yml ansible playbook to update nagios.

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