Running a Ganeti cluster on Guix

The latest addition to Guix's ever-growing list of services is a little-known virtualization toolkit called Ganeti. Ganeti is designed to keep virtual machines running on a cluster of servers even in the event of hardware failures, and to make maintenance and recovery tasks easy.

It is comparable to tools such as Proxmox or oVirt, but has some distinctive features. One is that there is no GUI: third party ones exist, but are not currently packaged in Guix, so you are left with a rich command-line client and a fully featured remote API.

Another interesting feature is that installing Ganeti on its own leaves you no way to actually deploy any virtual machines. That probably sounds crazy, but stems from the fact that Ganeti is designed to be API-driven and automated, thus it comes with a OS API and users need to install one or more OS providers in addition to Ganeti. OS providers offer a declarative way to deploy virtual machine variants and should feel natural to Guix users. At the time of writing, the providers available in Guix are debootstrap for provisioning Debian- and Ubuntu-based VMs, and of course a Guix provider.

Finally Ganeti comes with a sophisticated instance allocation framework that efficiently packs virtual machines across a cluster while maintaining N+1 redundancy in case of a failover scenario. It can also make informed scheduling decisions based on various cluster tags, such as ensuring primary and secondary nodes are on different power distribution lines.

(Note: if you are looking for a way to run just a few virtual machines on your local computer, you are probably better off using libvirt or even a Childhurd as Ganeti is fairly heavyweight and requires a complicated networking setup.)

Preparing the configuration

With introductions out of the way, let's see how we can deploy a Ganeti cluster using Guix. For this tutorial we will create a two-node cluster and connect instances to the local network using an Open vSwitch bridge with no VLANs. We assume that each node has a single network interface named eth0 connected to the same network, and that a dedicated partition /dev/sdz3 is available for virtual machine storage. It is possible to store VMs on a number of other storage backends, but a dedicated drive (or rather LVM volume group) is necessary to use the DRBD integration to replicate VM disks.

We'll start off by defining a few helper services to create the Open vSwitch bridge and ensure the physical network interface is in the "up" state. Since Open vSwich stores the configuration in a database, you might as well run the equivalent ovs-vsctl commands on the host once and be done with it, but we do it through the configuration system to ensure we don't forget it in the future when adding or reinstalling nodes.

(use-modules (gnu)
             (gnu packages linux)
             (gnu packages networking)
             (gnu services shepherd))

(define (start-interface if)
  #~(let ((ip #$(file-append iproute "/sbin/ip")))
      (invoke/quiet ip "link" "set" #$if "up")))

(define (stop-interface if)
  #~(let ((ip #$(file-append iproute "/sbin/ip")))
      (invoke/quiet ip "link" "set" #$if "down")))

;; This service is necessary to ensure eth0 is in the "up" state on boot
;; since it is otherwise unmanaged from Guix PoV.
(define (ifup-service if)
  (let ((name (string-append "ifup-" if)))
    (simple-service name shepherd-root-service-type
                    (list (shepherd-service
                           (provision (list (string->symbol name)))
                           (start #~(lambda ()
                                      #$(start-interface if)))
                           (stop #~(lambda (_)
                                     #$(stop-interface if)))
                           (respawn? #f))))))

;; Note: Remove vlan_mode to use tagged VLANs.
(define (create-openvswitch-bridge bridge uplink)
  #~(let ((ovs-vsctl (lambda (cmd)
                       (apply invoke/quiet
                              #$(file-append openvswitch "/bin/ovs-vsctl")
                              (string-tokenize cmd)))))
      (and (ovs-vsctl (string-append "--may-exist add-br " #$bridge))
           (ovs-vsctl (string-append "--may-exist add-port " #$bridge " "
                                     " vlan_mode=native-untagged")))))

(define (create-openvswitch-internal-port bridge port)
  #~(invoke/quiet #$(file-append openvswitch "/bin/ovs-vsctl")
                  "--may-exist" "add-port" #$bridge #$port
                  "--" "set" "Interface" #$port "type=internal"))

(define %openvswitch-configuration-service
  (simple-service 'openvswitch-configuration shepherd-root-service-type
                  (list (shepherd-service
                         (provision '(openvswitch-configuration))
                         (requirement '(vswitchd))
                         (start #~(lambda ()
                                       "br0" "eth0")
                                       "br0" "gnt0")))
                         (respawn? #f)))))

This defines a openvswitch-configuration service object that creates a logical switch br0, connects eth0 as the "uplink", and creates a logical port gnt0 that we will use later as the main network interface for this system. We also create an ifup service that can bring network interfaces up and down. By themselves these variables do nothing, we also have to add them to our operating-system configuration below.

Such a configuration might be suitable for a small home network. In a datacenter deployment you would likely use tagged VLANs, and maybe a traditional Linux bridge instead of Open vSwitch. You can also forego bridging altogether with a routed networking setup, or do any combination of the three.

With this in place, we can start creating the operating-system configuration that we will use for the Ganeti servers:

;; [continued from the above configuration snippet]

(use-service-modules base ganeti linux networking ssh)

  (host-name "node1")
  ;; Ganeti requires that each node and the cluster name resolves to an
  ;; IP address.  The easiest way to achieve this is by adding everything
  ;; to the hosts file.
  (hosts-file (plain-file "hosts" "       localhost
::1             localhost   ganeti.lan   node1   node2
   (append %default-kernel-arguments
           '(;; Disable DRBDs usermode helper, as Ganeti is the only entity
             ;; that should manage DRBD.

  (packages (append (map specification->package
                         '("qemu" "drbd-utils" "lvm2"

  (services (cons* (service ganeti-service-type
                             (file-storage-paths '("/srv/ganeti/file-storage"))
                              (list (guix-os %default-guix-variants)
                                     (list (debootstrap-variant
                                             (suite "buster")))
                                             (suite "testing")
                                               #:recursive? #t))
                                              (delete "linux-image-amd64"
                                             (components '("main" "contrib"))))))))))

                   ;; Ensure the DRBD kernel module is loaded.
                   (service kernel-module-loader-service-type

                   ;; Create a static IP on the "gnt0" Open vSwitch interface.
                   (service openvswitch-service-type)
                   (ifup-service "eth0")
                   (static-networking-service "gnt0" ""
                                              #:netmask ""
                                              #:gateway ""
                                              #:requirement '(openvswitch-configuration)
                                              #:name-servers '(""))

                   (service openssh-service-type
                             (permit-root-login 'without-password)))

Here we declare two OS "variants" for the debootstrap OS provider. Debootstrap variants rely on a set of scripts (known as "hooks") in the installation process to do things like configure networking, install bootloader, create users, etc. In the example above, the "buster" variant uses the default hooks provided by Guix which configures network and GRUB, whereas the "testing+contrib+paravirtualized" variant use a local directory next to the configuration file named "paravirt-hooks" (it is copied into the final system closure).

We also declare a default guix-os variant provided by Guix's Ganeti service.

Ganeti veterans may be surprised that each OS variant has its own hooks. The Ganeti deployments I've seen use a single set of hooks for all variants, sometimes with additional logic inside the script based on the variant. Guix offers a powerful abstraction that makes it trivial to create per-variant hooks, obsoleting the need for a big /etc/ganeti/instance-debootstrap/hooks directory. Of course you can still create it if you wish and set the hooks property of the variants to #f.

Not all Ganeti options are exposed in the configuration system yet. If you find it limiting, you can add custom files using extra-special-file, or ideally extend the <ganeti-configuration> data type to suite your needs. You can also use gnt-cluster copyfile and gnt-cluster command to distribute files or run executables, but undeclared changes in /etc may be lost on the next reboot or reconfigure.

Initializing a cluster

At this stage, you should run guix system reconfigure with the new configuration on all nodes that will participate in the cluster. If you do this over SSH or with guix deploy, beware that eth0 will lose network connectivity once it is "plugged in to" the virtual switch, and you need to add any IP configuration to gnt0.

The Guix configuration system does not currently support declaring LVM volume groups, so we will create these manually on each node. We could write our own declarative configuration like the %openvswitch-configuration-service, but for brevity and safety reasons we'll do it "by hand":

pvcreate /dev/sdz3
vgcreate ganetivg /dev/sdz3

On the node that will act as the "master node", run the init command:

Warning: this will create new SSH keypairs, both host keys and for the root user! You can prevent that by adding --no-ssh-init, but then you will need to distribute /var/lib/ganeti/known_hosts to all hosts, and authorize the Ganeti key for the root user in openssh-configuration. Here we let Ganeti manage the keys for simplicity. As a bonus, we can automatically rotate the cluster keys in the future using gnt-cluster renew-crypto --new-ssh-keys.

gnt-cluster init \
    --master-netdev=gnt0 \
    --vg-name=ganetivg \
    --enabled-disk-templates=file,plain,drbd \
    --drbd-usermode-helper=/run/current-system/profile/bin/true \
    --enabled-hypervisors=kvm \
    --hypervisor-parameters=kvm:kvm_flag=enabled \
    --nic-parameters=mode=openvswitch,link=br0 \
    --no-etc-hosts \

--no-etc-hosts prevents Ganeti from automatically updating the /etc/hosts file when nodes are added or removed, which makes little sense on Guix because it is recreated every reboot/reconfigure.

See the gnt-cluster manual for information on the available options. Most can be changed at runtime with gnt-cluster modify.

If all goes well, the command returns no output and you should have the ganeti.lan IP address visible on gnt0. You can run gnt-cluster verify to check that the cluster is in good shape. Most likely it complains about something:

root@node1 ~# gnt-cluster verify
Submitted jobs 3, 4
Waiting for job 3 ...
Thu Jul 16 18:26:34 2020 * Verifying cluster config
Thu Jul 16 18:26:34 2020 * Verifying cluster certificate files
Thu Jul 16 18:26:34 2020 * Verifying hypervisor parameters
Thu Jul 16 18:26:34 2020 * Verifying all nodes belong to an existing group
Waiting for job 4 ...
Thu Jul 16 18:26:34 2020 * Verifying group 'default'
Thu Jul 16 18:26:34 2020 * Gathering data (1 nodes)
Thu Jul 16 18:26:34 2020 * Gathering information about nodes (1 nodes)
Thu Jul 16 18:26:35 2020 * Gathering disk information (1 nodes)
Thu Jul 16 18:26:35 2020 * Verifying configuration file consistency
Thu Jul 16 18:26:35 2020 * Verifying node status
Thu Jul 16 18:26:35 2020   - ERROR: node node1: hypervisor kvm parameter verify failure (source cluster): Parameter 'kernel_path' fails validation: not found or not a file (current value: '/boot/vmlinuz-3-kvmU')
Thu Jul 16 18:26:35 2020 * Verifying instance status
Thu Jul 16 18:26:35 2020 * Verifying orphan volumes
Thu Jul 16 18:26:35 2020 * Verifying N+1 Memory redundancy
Thu Jul 16 18:26:35 2020 * Other Notes
Thu Jul 16 18:26:35 2020 * Hooks Results

When using the KVM hypervisor, Ganeti expects to find a dedicated kernel image for virtual machines in /boot. For this tutorial we only use fully virtualized instances (meaning each VM runs its own kernel), so we can set kernel_path to an empty string to make the warning disappear:

gnt-cluster modify -H kvm:kernel_path=

Now let's add our other machine to the cluster:

gnt-node add node2

Ganeti will log into the node, copy the cluster configuration and start the relevant Shepherd services. You may need to authorize node1's SSH key first. Run gnt-cluster verify again to check that everything is in order:

gnt-cluster verify

If you used --no-ssh-init earlier you will likely get SSH host key warnings here. In that case you should update /var/lib/ganeti/known_hosts with the new node information, and distribute it with gnt-cluster copyfile or by adding it to the OS configuration.

The above configuration will make three operating systems available:

# gnt-os list

Let's try them out. But first we'll make Ganeti aware of our network so it can choose a static IP for the virtual machines.

# gnt-network add --network= --gateway= lan
# gnt-network connect -N mode=openvswitch,link=br0 lan

Now we can add an instance:

root@node1 ~# gnt-instance add --no-name-check --no-ip-check \
    -o debootstrap+buster -t drbd --disk 0:size=5G \
    --net 0:network=lan,ip=pool bustervm1
Thu Jul 16 18:28:58 2020  - INFO: Selected nodes for instance bustervm1 via iallocator hail: node1, node2
Thu Jul 16 18:28:58 2020  - INFO: NIC/0 inherits netparams ['br0', 'openvswitch', '']
Thu Jul 16 18:28:58 2020  - INFO: Chose IP from network lan
Thu Jul 16 18:28:58 2020 * creating instance disks...
Thu Jul 16 18:29:03 2020 adding instance bustervm1 to cluster config
Thu Jul 16 18:29:03 2020 adding disks to cluster config
Thu Jul 16 18:29:03 2020  - INFO: Waiting for instance bustervm1 to sync disks
Thu Jul 16 18:29:03 2020  - INFO: - device disk/0:  0.60% done, 5m 26s remaining (estimated)
Thu Jul 16 18:31:08 2020  - INFO: - device disk/0: 100.00% done, 0s remaining (estimated)
Thu Jul 16 18:31:08 2020  - INFO: Instance bustervm1's disks are in sync
Thu Jul 16 18:31:08 2020  - INFO: Waiting for instance bustervm1 to sync disks
Thu Jul 16 18:31:08 2020  - INFO: Instance bustervm1's disks are in sync
Thu Jul 16 18:31:08 2020 * running the instance OS create scripts...
Thu Jul 16 18:32:09 2020 * starting instance...

Ganeti will automatically select the optimal primary and secondary node for this VM based on available cluster resources. You can manually specify primary and secondary nodes with the -n and -s options.

By default Ganeti assumes that the new instance is already configured in DNS, so we need --no-name-check and --no-ip-check to bypass some sanity tests.

Try adding another instance, now using the Guix OS provider with the 'plain' (LVM) disk backend:

gnt-instance add --no-name-check --no-ip-check -o guix+default \
    -t plain --disk 0:size=5G -B memory=1G,vcpus=2 \
    --net 0:network=lan,ip=pool \

The guix+default variant has a configuration that starts an SSH server and authorizes the hosts SSH key, and configures static networking based on information from Ganeti. To use other configuration files, you should declare variants with the config file as the configuration property. The Guix provider also supports "OS parameters" that lets you specify a specific Guix commit or branch:

gnt-instance add --no-name-check --no-ip-check \
    -o guix+gnome -O "commit=<commit>" \
    -H kvm:spice_bind=,cpu_type=host \
    -t file --file-storage-dir=/srv/ganeti/file-storage \
    --disk 0:size=20G -B minmem=1G,maxmem=6G,vcpus=3 \
    --net 0:network=lan,ip=pool -n node1 \

You can connect to a VM serial console using gnt-instance console <instance>. For this last VM we used a hypothetical 'guix+gnome' variant, and added a graphical SPICE console that you can connect to remotely using the spicy command.

If you are new to Ganeti, the next steps is to familiarize yourself with the gnt- family commands. Fun stuff to do include gnt-instance migrate to move VMs between hosts, gnt-node evacuate to migrate all VMs off a node, and gnt-cluster master-failover to move the master role to a different node.

If you wish to start over for any reason, you can use gnt-cluster destroy.

Final remarks

The declarative nature of Guix maps well to Ganetis OS API. OS variants can be composed and inherit from each other, something that is not easily achieved with traditional configuration management tools. The author had a lot of fun creating native data types in the Guix configuration system for Ganetis OS configuration, and it made me wonder whether other parts of Ganeti could be made declarative such as aspects of instance and cluster configuration. In any case I'm happy and excited to finally be able to use Guix as a Ganeti host OS.

Like most services in Guix, Ganeti comes with a system test that runs in a VM and ensures that things like initializing a cluster work. The continuous integration system runs this automatically whenever a dependency is updated, and provides comfort that both the package and service is in a good shape. Currently it has rudimentary service tests, but it can conceivably be extended to provision a real cluster inside Ganeti and try things like master-failover and live migration.

So far only the KVM hypervisor has been tested. If you use LXC or Xen with Ganeti, please reach out to and share your experience.

About GNU Guix

GNU Guix is a transactional package manager and an advanced distribution of the GNU system that respects user freedom. Guix can be used on top of any system running the kernel Linux, or it can be used as a standalone operating system distribution for i686, x86_64, ARMv7, and AArch64 machines.

In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection. When used as a standalone GNU/Linux distribution, Guix offers a declarative, stateless approach to operating system configuration management. Guix is highly customizable and hackable through Guile programming interfaces and extensions to the Scheme language.

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