Ganeti terminology

This section provides a small introduction to Ganeti terminology, which might be useful to read the rest of the document.

Cluster

A set of machines (nodes) that cooperate to offer a coherent highly available virtualization service.

Node

A physical machine which is member of a cluster. Nodes are the basic cluster infrastructure, and are not fault tolerant.

Master node

The node which controls the Cluster, from which all Ganeti commands must be given.

Instance

A virtual machine which runs on a cluster. It can be a fault tolerant highly available entity.

Pool

A pool is a set of clusters sharing the same network.

Meta-Cluster

Anything that concerns more than one cluster.

Prerequisites

You need to have your Ganeti cluster installed and configured before you try any of the commands in this document. Please follow the Ganeti installation tutorial for instructions on how to do that.

Adding/Removing an instance

Adding a new virtual instance to your Ganeti cluster is really easy. The command is:

gnt-instance add \
  -n TARGET_NODE:SECONDARY_NODE -o OS_TYPE -t DISK_TEMPLATE \
  INSTANCE_NAME

The instance name must be resolvable (e.g. exist in DNS) and usually to an address in the same subnet as the cluster itself. Options you can give to this command include:

• The disk size (-s) for a single-disk instance, or multiple --disk N:size=SIZE options for multi-instance disks
• The memory size (-B memory)
• The number of virtual CPUs (-B vcpus)
• Arguments for the NICs of the instance; by default, a single-NIC instance is created. The IP and/or bridge of the NIC can be changed via --nic 0:ip=IP,bridge=BRIDGE

There are four types of disk template you can choose from:

diskless

The instance has no disks. Only used for special purpouse operating systems or for testing.

file

The instance will use plain files as backend for its disks. No redundancy is provided, and this is somewhat more difficult to configure for high performance.

plain

The instance will use LVM devices as backend for its disks. No redundancy is provided.

drbd

Note

This is only valid for multi-node clusters using DRBD 8.0.x

A mirror is set between the local node and a remote one, which must be specified with the second value of the --node option. Use this option to obtain a highly available instance that can be failed over to a remote node should the primary one fail.

For example if you want to create an highly available instance use the drbd disk templates:

gnt-instance add -n TARGET_NODE:SECONDARY_NODE -o OS_TYPE -t drbd \
  INSTANCE_NAME

To know which operating systems your cluster supports you can use the command:

gnt-os list

Removing an instance is even easier than creating one. This operation is irrereversible and destroys all the contents of your instance. Use with care:

gnt-instance remove INSTANCE_NAME

Starting/Stopping an instance

Instances are automatically started at instance creation time. To manually start one which is currently stopped you can run:

gnt-instance startup INSTANCE_NAME

While the command to stop one is:

gnt-instance shutdown INSTANCE_NAME

The command to see all the instances configured and their status is:

gnt-instance list

Do not use the Xen commands to stop instances. If you run for example xm shutdown or xm destroy on an instance Ganeti will automatically restart it (via the ganeti-watcher).

Exporting/Importing an instance

You can create a snapshot of an instance disk and Ganeti configuration, which then you can backup, or import into another cluster. The way to export an instance is:

gnt-backup export -n TARGET_NODE INSTANCE_NAME

The target node can be any node in the cluster with enough space under /srv/ganeti to hold the instance image. Use the --noshutdown option to snapshot an instance without rebooting it. Any previous snapshot of the same instance existing cluster-wide under /srv/ganeti will be removed by this operation: if you want to keep them move them out of the Ganeti exports directory.

Importing an instance is similar to creating a new one. The command is:

gnt-backup import -n TARGET_NODE -t DISK_TEMPLATE \
  --src-node=NODE --src-dir=DIR INSTANCE_NAME

Most of the options available for the command gnt-instance add are supported here too.

Failing over an instance

If an instance is built in highly available mode you can at any time fail it over to its secondary node, even if the primary has somehow failed and it's not up anymore. Doing it is really easy, on the master node you can just run:

gnt-instance failover INSTANCE_NAME

That's it. After the command completes the secondary node is now the primary, and vice versa.

Live migrating an instance

If an instance is built in highly available mode, it currently runs and both its nodes are running fine, you can at migrate it over to its secondary node, without dowtime. On the master node you need to run:

gnt-instance migrate INSTANCE_NAME

Replacing an instance disks

So what if instead the secondary node for an instance has failed, or you plan to remove a node from your cluster, and you failed over all its instances, but it's still secondary for some? The solution here is to replace the instance disks, changing the secondary node:

gnt-instance replace-disks -n NODE INSTANCE_NAME

This process is a bit long, but involves no instance downtime, and at the end of it the instance has changed its secondary node, to which it can if necessary be failed over.

Failing over the master node

This is all good as long as the Ganeti Master Node is up. Should it go down, or should you wish to decommission it, just run on any other node the command:

gnt-cluster masterfailover

and the node you ran it on is now the new master.

Adding/Removing nodes

And of course, now that you know how to move instances around, it's easy to free up a node, and then you can remove it from the cluster:

gnt-node remove NODE_NAME

and maybe add a new one:

gnt-node add --secondary-ip=ADDRESS NODE_NAME

Accessing an instance's disks

From an instance's primary node you have access to its disks. Never ever mount the underlying logical volume manually on a fault tolerant instance, or you risk breaking replication. The correct way to access them is to run the command:

gnt-instance activate-disks INSTANCE_NAME

And then access the device that gets created. After you've finished you can deactivate them with the deactivate-disks command, which works in the same way.

Accessing an instance's console

The command to access a running instance's console is:

gnt-instance console INSTANCE_NAME

Use the console normally and then type ^] when done, to exit.

Instance OS definitions Debugging

Should you have any problems with operating systems support the command to ran to see a complete status for all your nodes is:

gnt-os diagnose

Cluster-wide debugging

The gnt-cluster command offers several options to run tests or execute cluster-wide operations. For example:

gnt-cluster command
gnt-cluster copyfile
gnt-cluster verify
gnt-cluster verify-disks
gnt-cluster getmaster
gnt-cluster version

See the man page gnt-cluster to know more about their usage.