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
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 " #$bridge " " #$uplink " vlan_mode=native-untagged"))))) (define (create-openvswitch-internal-port bridge port) #~(invoke/quiet #$(file-append openvswitch "/bin/ovs-vsctl") "--may-exist" "add-port" #$bridge #$port "vlan_mode=native-untagged" "--" "set" "Interface" #$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 () #$(create-openvswitch-bridge "br0" "eth0") #$(create-openvswitch-internal-port "br0" "gnt0"))) (respawn? #f)))))
This defines a
openvswitch-configuration service object that creates a
eth0 as the "uplink", and creates a logical
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
routed networking setup, or do any combination of the three.
With this in place, we can start creating the
that we will use for the Ganeti servers:
;; [continued from the above configuration snippet] (use-service-modules base ganeti linux networking ssh) (operating-system (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" " 127.0.0.1 localhost ::1 localhost 192.168.1.200 ganeti.lan 192.168.1.201 node1 192.168.1.202 node2 ")) (kernel-arguments (append %default-kernel-arguments '(;; Disable DRBDs usermode helper, as Ganeti is the only entity ;; that should manage DRBD. "drbd.usermode_helper=/run/current-system/profile/bin/true"))) (packages (append (map specification->package '("qemu" "drbd-utils" "lvm2" "ganeti-instance-guix" "ganeti-instance-debootstrap")) %base-packages)) (services (cons* (service ganeti-service-type (ganeti-configuration (file-storage-paths '("/srv/ganeti/file-storage")) (os (list (guix-os %default-guix-variants) (debootstrap-os (list (debootstrap-variant "buster" (debootstrap-configuration (suite "buster"))) (debootstrap-variant "testing+contrib+paravirtualized" (debootstrap-configuration (suite "testing") (hooks (local-file "paravirt-hooks" #:recursive? #t)) (extra-pkgs (delete "linux-image-amd64" %default-debootstrap-extra-pkgs)) (components '("main" "contrib")))))))))) ;; Ensure the DRBD kernel module is loaded. (service kernel-module-loader-service-type '("drbd")) ;; Create a static IP on the "gnt0" Open vSwitch interface. (service openvswitch-service-type) %openvswitch-configuration-service (ifup-service "eth0") (static-networking-service "gnt0" "192.168.1.201" #:netmask "255.255.255.0" #:gateway "192.168.1.1" #:requirement '(openvswitch-configuration) #:name-servers '("192.168.1.1")) (service openssh-service-type (openssh-configuration (permit-root-login 'without-password))) %base-services)))
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
Of course you can still create it if you wish and set the
hooks property of the
Not all Ganeti options are exposed in the configuration system yet. If you
find it limiting, you can add custom files using
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
eth0 will lose network connectivity once it is "plugged in to"
the virtual switch, and you need to add any IP configuration to
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
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
/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 \ ganeti.lan
--no-etc-hosts prevents Ganeti from automatically updating the
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
If all goes well, the command returns no output and you should have the
ganeti.lan IP address visible on
gnt0. You can run
to check that the cluster is in good shape. Most likely it complains about
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.
gnt-cluster verify again to check that everything is in order:
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
The above configuration will make three operating systems available:
# gnt-os list Name debootstrap+buster debootstrap+testing+contrib+paravirtualized guix+default
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=192.168.1.0/24 --gateway=192.168.1.1 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 192.168.1.2 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
By default Ganeti assumes that the new instance is already configured in DNS,
so we need
--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 \ guix1
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=0.0.0.0,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 \ guix2
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
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
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
email@example.com 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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