Synnefo » snf-ganeti » ganeti-local

Ganeti administrator's guide

============================

Documents Ganeti version |version|

.. contents::

.. highlight:: shell-example

Introduction

------------

Ganeti is a virtualization cluster management software. You are expected

to be a system administrator familiar with your Linux distribution and

the Xen or KVM virtualization environments before using it.

The various components of Ganeti all have man pages and interactive

help. This manual though will help you getting familiar with the system

by explaining the most common operations, grouped by related use.

After a terminology glossary and a section on the prerequisites needed

to use this manual, the rest of this document is divided in sections

for the different targets that a command affects: instance, nodes, etc.

.. _terminology-label:

Ganeti terminology

++++++++++++++++++

This section provides a small introduction to Ganeti terminology, which

might be useful when reading the rest of the document.

Cluster

~~~~~~~

A set of machines (nodes) that cooperate to offer a coherent, highly

available virtualization service under a single administration domain.

Node

~~~~

A physical machine which is member of a cluster.  Nodes are the basic

cluster infrastructure, and they don't need to be fault tolerant in

order to achieve high availability for instances.

Node can be added and removed (if they host no instances) at will from

the cluster. In a HA cluster and only with HA instances, the loss of any

single node will not cause disk data loss for any instance; of course,

a node crash will cause the crash of its primary instances.

A node belonging to a cluster can be in one of the following roles at a

given time:

- *master* node, which is the node from which the cluster is controlled

- *master candidate* node, only nodes in this role have the full cluster

  configuration and knowledge, and only master candidates can become the

  master node

- *regular* node, which is the state in which most nodes will be on

  bigger clusters (>20 nodes)

- *drained* node, nodes in this state are functioning normally but the

  cannot receive new instances; the intention is that nodes in this role

  have some issue and they are being evacuated for hardware repairs

- *offline* node, in which there is a record in the cluster

  configuration about the node, but the daemons on the master node will

  not talk to this node; any instances declared as having an offline

  node as either primary or secondary will be flagged as an error in the

  cluster verify operation

Depending on the role, each node will run a set of daemons:

- the :command:`ganeti-noded` daemon, which controls the manipulation of

  this node's hardware resources; it runs on all nodes which are in a

  cluster

- the :command:`ganeti-confd` daemon (Ganeti 2.1+) which runs on all

  nodes, but is only functional on master candidate nodes; this daemon

  can be disabled at configuration time if you don't need its

  functionality

- the :command:`ganeti-rapi` daemon which runs on the master node and

  offers an HTTP-based API for the cluster

- the :command:`ganeti-masterd` daemon which runs on the master node and

  allows control of the cluster

Beside the node role, there are other node flags that influence its

behaviour:

- the *master_capable* flag denotes whether the node can ever become a

  master candidate; setting this to 'no' means that auto-promotion will

  never make this node a master candidate; this flag can be useful for a

  remote node that only runs local instances, and having it become a

  master is impractical due to networking or other constraints

- the *vm_capable* flag denotes whether the node can host instances or

  not; for example, one might use a non-vm_capable node just as a master

  candidate, for configuration backups; setting this flag to no

  disallows placement of instances of this node, deactivates hypervisor

  and related checks on it (e.g. bridge checks, LVM check, etc.), and

  removes it from cluster capacity computations

Instance

~~~~~~~~

A virtual machine which runs on a cluster. It can be a fault tolerant,

highly available entity.

An instance has various parameters, which are classified in three

categories: hypervisor related-parameters (called ``hvparams``), general

parameters (called ``beparams``) and per network-card parameters (called

``nicparams``). All these parameters can be modified either at instance

level or via defaults at cluster level.

Disk template

~~~~~~~~~~~~~

The are multiple options for the storage provided to an instance; while

the instance sees the same virtual drive in all cases, the node-level

configuration varies between them.

There are five disk templates you can choose from:

diskless

  The instance has no disks. Only used for special purpose 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. Note that for security reasons the

  file storage directory must be listed under

  ``/etc/ganeti/file-storage-paths``, and that file is not copied

  automatically to all nodes by Ganeti.

sharedfile

  The instance will use plain files as backend, but Ganeti assumes that

  those files will be available and in sync automatically on all nodes.

  This allows live migration and failover of instances using this

  method. As for ``file`` the file storage directory must be listed under

  ``/etc/ganeti/file-storage-paths`` or ganeti will refuse to create

  instances under it.

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+

  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.

  .. note:: Ganeti does not support DRBD stacked devices:

     DRBD stacked setup is not fully symmetric and as such it is

     not working with live migration.

rbd

  The instance will use Volumes inside a RADOS cluster as backend for its

  disks. It will access them using the RADOS block device (RBD).

ext

  The instance will use an external storage provider. See

  :manpage:`ganeti-extstorage-interface(7)` for how to implement one.

IAllocator

~~~~~~~~~~

A framework for using external (user-provided) scripts to compute the

placement of instances on the cluster nodes. This eliminates the need to

manually specify nodes in instance add, instance moves, node evacuate,

etc.

In order for Ganeti to be able to use these scripts, they must be place

in the iallocator directory (usually ``lib/ganeti/iallocators`` under

the installation prefix, e.g. ``/usr/local``).

“Primary” and “secondary” concepts

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

An instance has a primary and depending on the disk configuration, might

also have a secondary node. The instance always runs on the primary node

and only uses its secondary node for disk replication.

Similarly, the term of primary and secondary instances when talking

about a node refers to the set of instances having the given node as

primary, respectively secondary.

Tags

~~~~

Tags are short strings that can be attached to either to cluster itself,

or to nodes or instances. They are useful as a very simplistic

information store for helping with cluster administration, for example

by attaching owner information to each instance after it's created::

  $ gnt-instance add … %instance1%

  $ gnt-instance add-tags %instance1% %owner:user2%

And then by listing each instance and its tags, this information could

be used for contacting the users of each instance.

Jobs and OpCodes

~~~~~~~~~~~~~~~~

While not directly visible by an end-user, it's useful to know that a

basic cluster operation (e.g. starting an instance) is represented

internally by Ganeti as an *OpCode* (abbreviation from operation

code). These OpCodes are executed as part of a *Job*. The OpCodes in a

single Job are processed serially by Ganeti, but different Jobs will be

processed (depending on resource availability) in parallel. They will

not be executed in the submission order, but depending on resource

availability, locks and (starting with Ganeti 2.3) priority. An earlier

job may have to wait for a lock while a newer job doesn't need any locks

and can be executed right away. Operations requiring a certain order

need to be submitted as a single job, or the client must submit one job

at a time and wait for it to finish before continuing.

For example, shutting down the entire cluster can be done by running the

command ``gnt-instance shutdown --all``, which will submit for each

instance a separate job containing the “shutdown instance” OpCode.

Prerequisites

+++++++++++++

You need to have your Ganeti cluster installed and configured before you

try any of the commands in this document. Please follow the

:doc:`install` for instructions on how to do that.

Instance management

-------------------

Adding an instance

++++++++++++++++++

The add operation might seem complex due to the many parameters it

accepts, but once you have understood the (few) required parameters and

the customisation capabilities you will see it is an easy operation.

The add operation requires at minimum five parameters:

- the OS for the instance

- the disk template

- the disk count and size

- the node specification or alternatively the iallocator to use

- and finally the instance name

The OS for the instance must be visible in the output of the command

``gnt-os list`` and specifies which guest OS to install on the instance.

The disk template specifies what kind of storage to use as backend for

the (virtual) disks presented to the instance; note that for instances

with multiple virtual disks, they all must be of the same type.

The node(s) on which the instance will run can be given either manually,

via the ``-n`` option, or computed automatically by Ganeti, if you have

installed any iallocator script.

With the above parameters in mind, the command is::

  $ gnt-instance add \

    -n %TARGET_NODE%:%SECONDARY_NODE% \

    -o %OS_TYPE% \

    -t %DISK_TEMPLATE% -s %DISK_SIZE% \

    %INSTANCE_NAME%

The instance name must be resolvable (e.g. exist in DNS) and usually

points to an address in the same subnet as the cluster itself.

The above command has the minimum required options; other options you

can give include, among others:

- The maximum/minimum memory size (``-B maxmem``, ``-B minmem``)

  (``-B memory`` can be used to specify only one size)

- 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 ``--net 0:ip=IP,link=BRIDGE``

See :manpage:`ganeti-instance(8)` for the detailed option list.

For example if you want to create an highly available instance, with a

single disk of 50GB and the default memory size, having primary node

``node1`` and secondary node ``node3``, use the following command::

  $ gnt-instance add -n node1:node3 -o debootstrap -t drbd -s 50G \

    instance1

There is a also a command for batch instance creation from a

specification file, see the ``batch-create`` operation in the

gnt-instance manual page.

Regular instance operations

+++++++++++++++++++++++++++

Removal

~~~~~~~

Removing an instance is even easier than creating one. This operation is

irreversible and destroys all the contents of your instance. Use with

care::

  $ gnt-instance remove %INSTANCE_NAME%

.. _instance-startup-label:

Startup/shutdown

~~~~~~~~~~~~~~~~

Instances are automatically started at instance creation time. To

manually start one which is currently stopped you can run::

  $ gnt-instance startup %INSTANCE_NAME%

Ganeti will start an instance with up to its maximum instance memory. If

not enough memory is available Ganeti will use all the available memory

down to the instance minimum memory. If not even that amount of memory

is free Ganeti will refuse to start the instance.

Note, that this will not work when an instance is in a permanently

stopped state ``offline``. In this case, you will first have to

put it back to online mode by running::

  $ gnt-instance modify --online %INSTANCE_NAME%

The command to stop the running instance is::

  $ gnt-instance shutdown %INSTANCE_NAME%

If you want to shut the instance down more permanently, so that it

does not require dynamically allocated resources (memory and vcpus),

after shutting down an instance, execute the following::

  $ gnt-instance modify --offline %INSTANCE_NAME%

.. warning:: Do not use the Xen or KVM commands directly to stop

   instances. If you run for example ``xm shutdown`` or ``xm destroy``

   on an instance Ganeti will automatically restart it (via

   the :command:`ganeti-watcher(8)` command which is launched via cron).

Querying instances

~~~~~~~~~~~~~~~~~~

There are two ways to get information about instances: listing

instances, which does a tabular output containing a given set of fields

about each instance, and querying detailed information about a set of

instances.

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

  $ gnt-instance list

The command can return a custom set of information when using the ``-o``

option (as always, check the manpage for a detailed specification). Each

instance will be represented on a line, thus making it easy to parse

this output via the usual shell utilities (grep, sed, etc.).

To get more detailed information about an instance, you can run::

  $ gnt-instance info %INSTANCE%

which will give a multi-line block of information about the instance,

it's hardware resources (especially its disks and their redundancy

status), etc. This is harder to parse and is more expensive than the

list operation, but returns much more detailed information.

Changing an instance's runtime memory

+++++++++++++++++++++++++++++++++++++

Ganeti will always make sure an instance has a value between its maximum

and its minimum memory available as runtime memory. As of version 2.6

Ganeti will only choose a size different than the maximum size when

starting up, failing over, or migrating an instance on a node with less

than the maximum memory available. It won't resize other instances in

order to free up space for an instance.

If you find that you need more memory on a node any instance can be

manually resized without downtime, with the command::

  $ gnt-instance modify -m %SIZE% %INSTANCE_NAME%

The same command can also be used to increase the memory available on an

instance, provided that enough free memory is available on its node, and

the specified size is not larger than the maximum memory size the

instance had when it was first booted (an instance will be unable to see

new memory above the maximum that was specified to the hypervisor at its

boot time, if it needs to grow further a reboot becomes necessary).

Export/Import

+++++++++++++

You can create a snapshot of an instance disk and its 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. Note that Ganeti

only keeps one snapshot for an instance - 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, you need to move

them out of the Ganeti exports directory.

Importing an instance is similar to creating a new one, but additionally

one must specify the location of the snapshot. The command is::

  $ gnt-backup import -n %TARGET_NODE% \

    --src-node=%NODE% --src-dir=%DIR% %INSTANCE_NAME%

By default, parameters will be read from the export information, but you

can of course pass them in via the command line - most of the options

available for the command :command:`gnt-instance add` are supported here

too.

Import of foreign instances

+++++++++++++++++++++++++++

There is a possibility to import a foreign instance whose disk data is

already stored as LVM volumes without going through copying it: the disk

adoption mode.

For this, ensure that the original, non-managed instance is stopped,

then create a Ganeti instance in the usual way, except that instead of

passing the disk information you specify the current volumes::

  $ gnt-instance add -t plain -n %HOME_NODE% ... \

    --disk 0:adopt=%lv_name%[,vg=%vg_name%] %INSTANCE_NAME%

This will take over the given logical volumes, rename them to the Ganeti

standard (UUID-based), and without installing the OS on them start

directly the instance. If you configure the hypervisor similar to the

non-managed configuration that the instance had, the transition should

be seamless for the instance. For more than one disk, just pass another

disk parameter (e.g. ``--disk 1:adopt=...``).

Instance kernel selection

+++++++++++++++++++++++++

The kernel that instances uses to bootup can come either from the node,

or from instances themselves, depending on the setup.

Xen-PVM

~~~~~~~

With Xen PVM, there are three options.

First, you can use a kernel from the node, by setting the hypervisor

parameters as such:

- ``kernel_path`` to a valid file on the node (and appropriately

  ``initrd_path``)

- ``kernel_args`` optionally set to a valid Linux setting (e.g. ``ro``)

- ``root_path`` to a valid setting (e.g. ``/dev/xvda1``)

- ``bootloader_path`` and ``bootloader_args`` to empty

Alternatively, you can delegate the kernel management to instances, and

use either ``pvgrub`` or the deprecated ``pygrub``. For this, you must

install the kernels and initrds in the instance and create a valid GRUB

v1 configuration file.

For ``pvgrub`` (new in version 2.4.2), you need to set:

- ``kernel_path`` to point to the ``pvgrub`` loader present on the node

  (e.g. ``/usr/lib/xen/boot/pv-grub-x86_32.gz``)

- ``kernel_args`` to the path to the GRUB config file, relative to the

  instance (e.g. ``(hd0,0)/grub/menu.lst``)

- ``root_path`` **must** be empty

- ``bootloader_path`` and ``bootloader_args`` to empty

While ``pygrub`` is deprecated, here is how you can configure it:

- ``bootloader_path`` to the pygrub binary (e.g. ``/usr/bin/pygrub``)

- the other settings are not important

More information can be found in the Xen wiki pages for `pvgrub

<http://wiki.xensource.com/xenwiki/PvGrub>`_ and `pygrub

<http://wiki.xensource.com/xenwiki/PyGrub>`_.

KVM

~~~

For KVM also the kernel can be loaded either way.

For loading the kernels from the node, you need to set:

- ``kernel_path`` to a valid value

- ``initrd_path`` optionally set if you use an initrd

- ``kernel_args`` optionally set to a valid value (e.g. ``ro``)

If you want instead to have the instance boot from its disk (and execute

its bootloader), simply set the ``kernel_path`` parameter to an empty

string, and all the others will be ignored.

Instance HA features

--------------------

.. note:: This section only applies to multi-node clusters

.. _instance-change-primary-label:

Changing the primary node

+++++++++++++++++++++++++

There are three ways to exchange an instance's primary and secondary

nodes; the right one to choose depends on how the instance has been

created and the status of its current primary node. See

:ref:`rest-redundancy-label` for information on changing the secondary

node. Note that it's only possible to change the primary node to the

secondary and vice-versa; a direct change of the primary node with a

third node, while keeping the current secondary is not possible in a

single step, only via multiple operations as detailed in

:ref:`instance-relocation-label`.

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.

The instance will be started with an amount of memory between its

``maxmem`` and its ``minmem`` value, depending on the free memory on its

target node, or the operation will fail if that's not possible. See

:ref:`instance-startup-label` for details.

If the instance's disk template is of type rbd, then you can specify

the target node (which can be any node) explicitly, or specify an

iallocator plugin. If you omit both, the default iallocator will be

used to determine the target node::

  $ gnt-instance failover -n %TARGET_NODE% %INSTANCE_NAME%

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 migrate it over to its

secondary node, without downtime. On the master node you need to run::

  $ gnt-instance migrate %INSTANCE_NAME%

The current load on the instance and its memory size will influence how

long the migration will take. In any case, for both KVM and Xen

hypervisors, the migration will be transparent to the instance.

If the destination node has less memory than the instance's current

runtime memory, but at least the instance's minimum memory available

Ganeti will automatically reduce the instance runtime memory before

migrating it, unless the ``--no-runtime-changes`` option is passed, in

which case the target node should have at least the instance's current

runtime memory free.

If the instance's disk template is of type rbd, then you can specify

the target node (which can be any node) explicitly, or specify an

iallocator plugin. If you omit both, the default iallocator will be

used to determine the target node::

   $ gnt-instance migrate -n %TARGET_NODE% %INSTANCE_NAME%

Moving an instance (offline)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If an instance has not been create as mirrored, then the only way to

change its primary node is to execute the move command::

  $ gnt-instance move -n %NEW_NODE% %INSTANCE%

This has a few prerequisites:

- the instance must be stopped

- its current primary node must be on-line and healthy

- the disks of the instance must not have any errors

Since this operation actually copies the data from the old node to the

new node, expect it to take proportional to the size of the instance's

disks and the speed of both the nodes' I/O system and their networking.

Disk operations

+++++++++++++++

Disk failures are a common cause of errors in any server

deployment. Ganeti offers protection from single-node failure if your

instances were created in HA mode, and it also offers ways to restore

redundancy after a failure.

Preparing for disk operations

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is important to note that for Ganeti to be able to do any disk

operation, the Linux machines on top of which Ganeti runs must be

consistent; for LVM, this means that the LVM commands must not return

failures; it is common that after a complete disk failure, any LVM

command aborts with an error similar to::

  $ vgs

  /dev/sdb1: read failed after 0 of 4096 at 0: Input/output error

  /dev/sdb1: read failed after 0 of 4096 at 750153695232: Input/output error

  /dev/sdb1: read failed after 0 of 4096 at 0: Input/output error

  Couldn't find device with uuid 't30jmN-4Rcf-Fr5e-CURS-pawt-z0jU-m1TgeJ'.

  Couldn't find all physical volumes for volume group xenvg.

Before restoring an instance's disks to healthy status, it's needed to

fix the volume group used by Ganeti so that we can actually create and

manage the logical volumes. This is usually done in a multi-step

process:

#. first, if the disk is completely gone and LVM commands exit with

   “Couldn't find device with uuid…” then you need to run the command::

    $ vgreduce --removemissing %VOLUME_GROUP%

#. after the above command, the LVM commands should be executing

   normally (warnings are normal, but the commands will not fail

   completely).

#. if the failed disk is still visible in the output of the ``pvs``

   command, you need to deactivate it from allocations by running::

    $ pvs -x n /dev/%DISK%

At this point, the volume group should be consistent and any bad

physical volumes should not longer be available for allocation.

Note that since version 2.1 Ganeti provides some commands to automate

these two operations, see :ref:`storage-units-label`.

.. _rest-redundancy-label:

Restoring redundancy for DRBD-based instances

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A DRBD instance has two nodes, and the storage on one of them has

failed. Depending on which node (primary or secondary) has failed, you

have three options at hand:

- if the storage on the primary node has failed, you need to re-create

  the disks on it

- if the storage on the secondary node has failed, you can either

  re-create the disks on it or change the secondary and recreate

  redundancy on the new secondary node

Of course, at any point it's possible to force re-creation of disks even

though everything is already fine.

For all three cases, the ``replace-disks`` operation can be used::

  # re-create disks on the primary node

  $ gnt-instance replace-disks -p %INSTANCE_NAME%

  # re-create disks on the current secondary

  $ gnt-instance replace-disks -s %INSTANCE_NAME%

  # change the secondary node, via manual specification

  $ gnt-instance replace-disks -n %NODE% %INSTANCE_NAME%

  # change the secondary node, via an iallocator script

  $ gnt-instance replace-disks -I %SCRIPT% %INSTANCE_NAME%

  # since Ganeti 2.1: automatically fix the primary or secondary node

  $ gnt-instance replace-disks -a %INSTANCE_NAME%

Since the process involves copying all data from the working node to the

target node, it will take a while, depending on the instance's disk

size, node I/O system and network speed. But it is (barring any network

interruption) completely transparent for the instance.

Re-creating disks for non-redundant instances

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 2.1

For non-redundant instances, there isn't a copy (except backups) to

re-create the disks. But it's possible to at-least re-create empty

disks, after which a reinstall can be run, via the ``recreate-disks``

command::

  $ gnt-instance recreate-disks %INSTANCE%

Note that this will fail if the disks already exists. The instance can

be assigned to new nodes automatically by specifying an iallocator

through the ``--iallocator`` option.

Conversion of an instance's disk type

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It is possible to convert between a non-redundant instance of type

``plain`` (LVM storage) and redundant ``drbd`` via the ``gnt-instance

modify`` command::

  # start with a non-redundant instance

  $ gnt-instance add -t plain ... %INSTANCE%

  # later convert it to redundant

  $ gnt-instance stop %INSTANCE%

  $ gnt-instance modify -t drbd -n %NEW_SECONDARY% %INSTANCE%

  $ gnt-instance start %INSTANCE%

  # and convert it back

  $ gnt-instance stop %INSTANCE%

  $ gnt-instance modify -t plain %INSTANCE%

  $ gnt-instance start %INSTANCE%

The conversion must be done while the instance is stopped, and

converting from plain to drbd template presents a small risk, especially

if the instance has multiple disks and/or if one node fails during the

conversion procedure). As such, it's recommended (as always) to make

sure that downtime for manual recovery is acceptable and that the

instance has up-to-date backups.

Debugging instances

+++++++++++++++++++

Accessing an instance's disks

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

From an instance's primary node you can have access to its disks. Never

ever mount the underlying logical volume manually on a fault tolerant

instance, or will break replication and your data will be

inconsistent. The correct way to access an instance's disks is to run

(on the master node, as usual) the command::

  $ gnt-instance activate-disks %INSTANCE%

And then, *on the primary node of the instance*, access the device that

gets created. For example, you could mount the given disks, then edit

files on the filesystem, etc.

Note that with partitioned disks (as opposed to whole-disk filesystems),

you will need to use a tool like :manpage:`kpartx(8)`::

  # on node1

  $ gnt-instance activate-disks %instance1%

  node3:disk/0:…

  $ ssh node3

  # on node 3

  $ kpartx -l /dev/…

  $ kpartx -a /dev/…

  $ mount /dev/mapper/… /mnt/

  # edit files under mnt as desired

  $ umount /mnt/

  $ kpartx -d /dev/…

  $ exit

  # back to node 1

After you've finished you can deactivate them with the deactivate-disks

command, which works in the same way::

  $ gnt-instance deactivate-disks %INSTANCE%

Note that if any process started by you is still using the disks, the

above command will error out, and you **must** cleanup and ensure that

the above command runs successfully before you start the instance,

otherwise the instance will suffer corruption.

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.

Other instance operations

+++++++++++++++++++++++++

Reboot

~~~~~~

There is a wrapper command for rebooting instances::

  $ gnt-instance reboot %instance2%

By default, this does the equivalent of shutting down and then starting

the instance, but it accepts parameters to perform a soft-reboot (via

the hypervisor), a hard reboot (hypervisor shutdown and then startup) or

a full one (the default, which also de-configures and then configures

again the disks of the instance).

Instance OS definitions debugging

+++++++++++++++++++++++++++++++++

Should you have any problems with instance operating systems the command

to see a complete status for all your nodes is::

   $ gnt-os diagnose

.. _instance-relocation-label:

Instance relocation

~~~~~~~~~~~~~~~~~~~

While it is not possible to move an instance from nodes ``(A, B)`` to

nodes ``(C, D)`` in a single move, it is possible to do so in a few

steps::

  # instance is located on A, B

  $ gnt-instance replace -n %nodeC% %instance1%

  # instance has moved from (A, B) to (A, C)

  # we now flip the primary/secondary nodes

  $ gnt-instance migrate %instance1%

  # instance lives on (C, A)

  # we can then change A to D via:

  $ gnt-instance replace -n %nodeD% %instance1%

Which brings it into the final configuration of ``(C, D)``. Note that we

needed to do two replace-disks operation (two copies of the instance

disks), because we needed to get rid of both the original nodes (A and

B).

Node operations

---------------

There are much fewer node operations available than for instances, but

they are equivalently important for maintaining a healthy cluster.

Add/readd

+++++++++

It is at any time possible to extend the cluster with one more node, by

using the node add operation::

  $ gnt-node add %NEW_NODE%

If the cluster has a replication network defined, then you need to pass

the ``-s REPLICATION_IP`` parameter to this option.

A variation of this command can be used to re-configure a node if its

Ganeti configuration is broken, for example if it has been reinstalled

by mistake::

  $ gnt-node add --readd %EXISTING_NODE%

This will reinitialise the node as if it's been newly added, but while

keeping its existing configuration in the cluster (primary/secondary IP,

etc.), in other words you won't need to use ``-s`` here.

Changing the node role

++++++++++++++++++++++

A node can be in different roles, as explained in the

:ref:`terminology-label` section. Promoting a node to the master role is

special, while the other roles are handled all via a single command.

Failing over the master node

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you want to promote a different node to the master role (for whatever

reason), run on any other master-candidate node the command::

  $ gnt-cluster master-failover

and the node you ran it on is now the new master. In case you try to run

this on a non master-candidate node, you will get an error telling you

which nodes are valid.

Changing between the other roles

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``gnt-node modify`` command can be used to select a new role::

  # change to master candidate

  $ gnt-node modify -C yes %NODE%

  # change to drained status

  $ gnt-node modify -D yes %NODE%

  # change to offline status

  $ gnt-node modify -O yes %NODE%

  # change to regular mode (reset all flags)

  $ gnt-node modify -O no -D no -C no %NODE%

Note that the cluster requires that at any point in time, a certain

number of nodes are master candidates, so changing from master candidate

to other roles might fail. It is recommended to either force the

operation (via the ``--force`` option) or first change the number of

master candidates in the cluster - see :ref:`cluster-config-label`.

Evacuating nodes

++++++++++++++++

There are two steps of moving instances off a node:

- moving the primary instances (actually converting them into secondary

  instances)

- moving the secondary instances (including any instances converted in

  the step above)

Primary instance conversion

~~~~~~~~~~~~~~~~~~~~~~~~~~~

For this step, you can use either individual instance move

commands (as seen in :ref:`instance-change-primary-label`) or the bulk

per-node versions; these are::

  $ gnt-node migrate %NODE%

  $ gnt-node evacuate -s %NODE%

Note that the instance “move” command doesn't currently have a node

equivalent.

Both these commands, or the equivalent per-instance command, will make

this node the secondary node for the respective instances, whereas their

current secondary node will become primary. Note that it is not possible

to change in one step the primary node to another node as primary, while

keeping the same secondary node.

Secondary instance evacuation

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For the evacuation of secondary instances, a command called

:command:`gnt-node evacuate` is provided and its syntax is::

  $ gnt-node evacuate -I %IALLOCATOR_SCRIPT% %NODE%

  $ gnt-node evacuate -n %DESTINATION_NODE% %NODE%

The first version will compute the new secondary for each instance in

turn using the given iallocator script, whereas the second one will

simply move all instances to DESTINATION_NODE.

Removal

+++++++

Once a node no longer has any instances (neither primary nor secondary),

it's easy to remove it from the cluster::

  $ gnt-node remove %NODE_NAME%

This will deconfigure the node, stop the ganeti daemons on it and leave

it hopefully like before it joined to the cluster.

Replication network changes

+++++++++++++++++++++++++++

The :command:`gnt-node modify -s` command can be used to change the

secondary IP of a node. This operation can only be performed if:

- No instance is active on the target node

- The new target IP is reachable from the master's secondary IP

Also this operation will not allow to change a node from single-homed

(same primary and secondary ip) to multi-homed (separate replication

network) or vice versa, unless:

- The target node is the master node and `--force` is passed.

- The target cluster is single-homed and the new primary ip is a change

  to single homed for a particular node.

- The target cluster is multi-homed and the new primary ip is a change

  to multi homed for a particular node.

For example to do a single-homed to multi-homed conversion::

  $ gnt-node modify --force -s %SECONDARY_IP% %MASTER_NAME%

  $ gnt-node modify -s %SECONDARY_IP% %NODE1_NAME%

  $ gnt-node modify -s %SECONDARY_IP% %NODE2_NAME%

  $ gnt-node modify -s %SECONDARY_IP% %NODE3_NAME%

  ...

The same commands can be used for multi-homed to single-homed except the

secondary IPs should be the same as the primaries for each node, for

that case.

Storage handling

++++++++++++++++

When using LVM (either standalone or with DRBD), it can become tedious

to debug and fix it in case of errors. Furthermore, even file-based

storage can become complicated to handle manually on many hosts. Ganeti

provides a couple of commands to help with automation.

Logical volumes

~~~~~~~~~~~~~~~

This is a command specific to LVM handling. It allows listing the

logical volumes on a given node or on all nodes and their association to

instances via the ``volumes`` command::

  $ gnt-node volumes

  Node  PhysDev   VG    Name             Size Instance

  node1 /dev/sdb1 xenvg e61fbc97-….disk0 512M instance17

  node1 /dev/sdb1 xenvg ebd1a7d1-….disk0 512M instance19

  node2 /dev/sdb1 xenvg 0af08a3d-….disk0 512M instance20

  node2 /dev/sdb1 xenvg cc012285-….disk0 512M instance16

  node2 /dev/sdb1 xenvg f0fac192-….disk0 512M instance18

The above command maps each logical volume to a volume group and

underlying physical volume and (possibly) to an instance.

.. _storage-units-label:

Generalized storage handling

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. versionadded:: 2.1

Starting with Ganeti 2.1, a new storage framework has been implemented

that tries to abstract the handling of the storage type the cluster

uses.

First is listing the backend storage and their space situation::

  $ gnt-node list-storage

  Node  Name        Size Used   Free

  node1 /dev/sda7 673.8G   0M 673.8G

  node1 /dev/sdb1 698.6G 1.5G 697.1G

  node2 /dev/sda7 673.8G   0M 673.8G

  node2 /dev/sdb1 698.6G 1.0G 697.6G

The default is to list LVM physical volumes. It's also possible to list

the LVM volume groups::

  $ gnt-node list-storage -t lvm-vg

  Node  Name  Size

  node1 xenvg 1.3T

  node2 xenvg 1.3T

Next is repairing storage units, which is currently only implemented for

volume groups and does the equivalent of ``vgreduce --removemissing``::

  $ gnt-node repair-storage %node2% lvm-vg xenvg

  Sun Oct 25 22:21:45 2009 Repairing storage unit 'xenvg' on node2 ...

Last is the modification of volume properties, which is (again) only

implemented for LVM physical volumes and allows toggling the

``allocatable`` value::

  $ gnt-node modify-storage --allocatable=no %node2% lvm-pv /dev/%sdb1%

Use of the storage commands

~~~~~~~~~~~~~~~~~~~~~~~~~~~

All these commands are needed when recovering a node from a disk

failure:

- first, we need to recover from complete LVM failure (due to missing

  disk), by running the ``repair-storage`` command

- second, we need to change allocation on any partially-broken disk

  (i.e. LVM still sees it, but it has bad blocks) by running

  ``modify-storage``

- then we can evacuate the instances as needed

Cluster operations

------------------

Beside the cluster initialisation command (which is detailed in the

:doc:`install` document) and the master failover command which is

explained under node handling, there are a couple of other cluster

operations available.

.. _cluster-config-label:

Standard operations

+++++++++++++++++++

One of the few commands that can be run on any node (not only the

master) is the ``getmaster`` command::

  # on node2

  $ gnt-cluster getmaster

  node1.example.com

It is possible to query and change global cluster parameters via the

``info`` and ``modify`` commands::

  $ gnt-cluster info

  Cluster name: cluster.example.com

  Cluster UUID: 07805e6f-f0af-4310-95f1-572862ee939c

  Creation time: 2009-09-25 05:04:15

  Modification time: 2009-10-18 22:11:47

  Master node: node1.example.com

  Architecture (this node): 64bit (x86_64)

  …

  Tags: foo

  Default hypervisor: xen-pvm

  Enabled hypervisors: xen-pvm

  Hypervisor parameters:

    - xen-pvm:

        root_path: /dev/sda1

        …

  Cluster parameters:

    - candidate pool size: 10

      …

  Default instance parameters:

    - default:

        memory: 128

        …

  Default nic parameters:

    - default:

        link: xen-br0

        …

There various parameters above can be changed via the ``modify``

commands as follows:

- the hypervisor parameters can be changed via ``modify -H

  xen-pvm:root_path=…``, and so on for other hypervisors/key/values

- the "default instance parameters" are changeable via ``modify -B

  parameter=value…`` syntax

- the cluster parameters are changeable via separate options to the

  modify command (e.g. ``--candidate-pool-size``, etc.)

For detailed option list see the :manpage:`gnt-cluster(8)` man page.

The cluster version can be obtained via the ``version`` command::

  $ gnt-cluster version

  Software version: 2.1.0

  Internode protocol: 20

  Configuration format: 2010000

  OS api version: 15

  Export interface: 0

This is not very useful except when debugging Ganeti.

Global node commands

++++++++++++++++++++

There are two commands provided for replicating files to all nodes of a

cluster and for running commands on all the nodes::

  $ gnt-cluster copyfile %/path/to/file%

  $ gnt-cluster command %ls -l /path/to/file%

These are simple wrappers over scp/ssh and more advanced usage can be

obtained using :manpage:`dsh(1)` and similar commands. But they are

useful to update an OS script from the master node, for example.

Cluster verification

++++++++++++++++++++

There are three commands that relate to global cluster checks. The first

one is ``verify`` which gives an overview on the cluster state,

highlighting any issues. In normal operation, this command should return

no ``ERROR`` messages::

  $ gnt-cluster verify

  Sun Oct 25 23:08:58 2009 * Verifying global settings

  Sun Oct 25 23:08:58 2009 * Gathering data (2 nodes)

  Sun Oct 25 23:09:00 2009 * Verifying node status

  Sun Oct 25 23:09:00 2009 * Verifying instance status

  Sun Oct 25 23:09:00 2009 * Verifying orphan volumes

  Sun Oct 25 23:09:00 2009 * Verifying remaining instances

  Sun Oct 25 23:09:00 2009 * Verifying N+1 Memory redundancy

  Sun Oct 25 23:09:00 2009 * Other Notes

  Sun Oct 25 23:09:00 2009   - NOTICE: 5 non-redundant instance(s) found.

  Sun Oct 25 23:09:00 2009 * Hooks Results

The second command is ``verify-disks``, which checks that the instance's

disks have the correct status based on the desired instance state

(up/down)::

  $ gnt-cluster verify-disks

Note that this command will show no output when disks are healthy.

The last command is used to repair any discrepancies in Ganeti's

recorded disk size and the actual disk size (disk size information is

needed for proper activation and growth of DRBD-based disks)::

  $ gnt-cluster repair-disk-sizes

  Sun Oct 25 23:13:16 2009  - INFO: Disk 0 of instance instance1 has mismatched size, correcting: recorded 512, actual 2048

  Sun Oct 25 23:13:17 2009  - WARNING: Invalid result from node node4, ignoring node results

The above shows one instance having wrong disk size, and a node which

returned invalid data, and thus we ignored all primary instances of that

node.

Configuration redistribution

++++++++++++++++++++++++++++

If the verify command complains about file mismatches between the master

and other nodes, due to some node problems or if you manually modified

configuration files, you can force an push of the master configuration

to all other nodes via the ``redist-conf`` command::

  $ gnt-cluster redist-conf

This command will be silent unless there are problems sending updates to

the other nodes.

Cluster renaming

++++++++++++++++

It is possible to rename a cluster, or to change its IP address, via the

``rename`` command. If only the IP has changed, you need to pass the

current name and Ganeti will realise its IP has changed::

  $ gnt-cluster rename %cluster.example.com%

  This will rename the cluster to 'cluster.example.com'. If

  you are connected over the network to the cluster name, the operation

  is very dangerous as the IP address will be removed from the node and

  the change may not go through. Continue?

  y/[n]/?: %y%

  Failure: prerequisites not met for this operation:

  Neither the name nor the IP address of the cluster has changed

In the above output, neither value has changed since the cluster

initialisation so the operation is not completed.

Queue operations

++++++++++++++++

The job queue execution in Ganeti 2.0 and higher can be inspected,

suspended and resumed via the ``queue`` command::

  $ gnt-cluster queue info

  The drain flag is unset

  $ gnt-cluster queue drain

  $ gnt-instance stop %instance1%

  Failed to submit job for instance1: Job queue is drained, refusing job

  $ gnt-cluster queue info

  The drain flag is set

  $ gnt-cluster queue undrain

This is most useful if you have an active cluster and you need to

upgrade the Ganeti software, or simply restart the software on any node:

#. suspend the queue via ``queue drain``

#. wait until there are no more running jobs via ``gnt-job list``

#. restart the master or another node, or upgrade the software

#. resume the queue via ``queue undrain``

.. note:: this command only stores a local flag file, and if you

   failover the master, it will not have effect on the new master.

Watcher control

+++++++++++++++

The :manpage:`ganeti-watcher(8)` is a program, usually scheduled via

``cron``, that takes care of cluster maintenance operations (restarting

downed instances, activating down DRBD disks, etc.). However, during

maintenance and troubleshooting, this can get in your way; disabling it

via commenting out the cron job is not so good as this can be

forgotten. Thus there are some commands for automated control of the

watcher: ``pause``, ``info`` and ``continue``::

  $ gnt-cluster watcher info

  The watcher is not paused.

  $ gnt-cluster watcher pause %1h%

  The watcher is paused until Mon Oct 26 00:30:37 2009.

  $ gnt-cluster watcher info

  The watcher is paused until Mon Oct 26 00:30:37 2009.

  $ ganeti-watcher -d

  2009-10-25 23:30:47,984:  pid=28867 ganeti-watcher:486 DEBUG Pause has been set, exiting

  $ gnt-cluster watcher continue

  The watcher is no longer paused.

  $ ganeti-watcher -d

  2009-10-25 23:31:04,789:  pid=28976 ganeti-watcher:345 DEBUG Archived 0 jobs, left 0

  2009-10-25 23:31:05,884:  pid=28976 ganeti-watcher:280 DEBUG Got data from cluster, writing instance status file

  2009-10-25 23:31:06,061:  pid=28976 ganeti-watcher:150 DEBUG Data didn't change, just touching status file

  $ gnt-cluster watcher info

  The watcher is not paused.

The exact details of the argument to the ``pause`` command are available

in the manpage.

.. note:: this command only stores a local flag file, and if you

   failover the master, it will not have effect on the new master.

Node auto-maintenance

+++++++++++++++++++++

If the cluster parameter ``maintain_node_health`` is enabled (see the

manpage for :command:`gnt-cluster`, the init and modify subcommands),

then the following will happen automatically:

- the watcher will shutdown any instances running on offline nodes

- the watcher will deactivate any DRBD devices on offline nodes

In the future, more actions are planned, so only enable this parameter

if the nodes are completely dedicated to Ganeti; otherwise it might be

possible to lose data due to auto-maintenance actions.

Removing a cluster entirely

+++++++++++++++++++++++++++

The usual method to cleanup a cluster is to run ``gnt-cluster destroy``

however if the Ganeti installation is broken in any way then this will

not run.

It is possible in such a case to cleanup manually most if not all traces

of a cluster installation by following these steps on all of the nodes:

1. Shutdown all instances. This depends on the virtualisation method

   used (Xen, KVM, etc.):

  - Xen: run ``xm list`` and ``xm destroy`` on all the non-Domain-0

    instances

  - KVM: kill all the KVM processes

  - chroot: kill all processes under the chroot mountpoints

2. If using DRBD, shutdown all DRBD minors (which should by at this time

   no-longer in use by instances); on each node, run ``drbdsetup

   /dev/drbdN down`` for each active DRBD minor.

3. If using LVM, cleanup the Ganeti volume group; if only Ganeti created

   logical volumes (and you are not sharing the volume group with the

   OS, for example), then simply running ``lvremove -f xenvg`` (replace

   'xenvg' with your volume group name) should do the required cleanup.

4. If using file-based storage, remove recursively all files and

   directories under your file-storage directory: ``rm -rf

   /srv/ganeti/file-storage/*`` replacing the path with the correct path

   for your cluster.

5. Stop the ganeti daemons (``/etc/init.d/ganeti stop``) and kill any

   that remain alive (``pgrep ganeti`` and ``pkill ganeti``).

6. Remove the ganeti state directory (``rm -rf /var/lib/ganeti/*``),

   replacing the path with the correct path for your installation.

7. If using RBD, run ``rbd unmap /dev/rbdN`` to unmap the RBD disks.

   Then remove the RBD disk images used by Ganeti, identified by their

   UUIDs (``rbd rm uuid.rbd.diskN``).

On the master node, remove the cluster from the master-netdev (usually

``xen-br0`` for bridged mode, otherwise ``eth0`` or similar), by running

``ip a del $clusterip/32 dev xen-br0`` (use the correct cluster ip and

network device name).

At this point, the machines are ready for a cluster creation; in case

you want to remove Ganeti completely, you need to also undo some of the

SSH changes and log directories:

- ``rm -rf /var/log/ganeti /srv/ganeti`` (replace with the correct

  paths)

- remove from ``/root/.ssh`` the keys that Ganeti added (check the

  ``authorized_keys`` and ``id_dsa`` files)

- regenerate the host's SSH keys (check the OpenSSH startup scripts)

- uninstall Ganeti

Otherwise, if you plan to re-create the cluster, you can just go ahead

and rerun ``gnt-cluster init``.

Monitoring the cluster

----------------------

Starting with Ganeti 2.8, a monitoring daemon is available, providing

information about the status and the performance of the system.

The monitoring daemon runs on every node, listening on TCP port 1815. Each

instance of the daemon provides information related to the node it is running

on.

.. include:: monitoring-query-format.rst

Tags handling

-------------

The tags handling (addition, removal, listing) is similar for all the

objects that support it (instances, nodes, and the cluster).

Limitations

+++++++++++

Note that the set of characters present in a tag and the maximum tag

length are restricted. Currently the maximum length is 128 characters,

there can be at most 4096 tags per object, and the set of characters is

comprised by alphanumeric characters and additionally ``.+*/:@-``.

Operations

++++++++++

Tags can be added via ``add-tags``::

  $ gnt-instance add-tags %INSTANCE% %a% %b% %c%

  $ gnt-node add-tags %INSTANCE% %a% %b% %c%

  $ gnt-cluster add-tags %a% %b% %c%

The above commands add three tags to an instance, to a node and to the

cluster. Note that the cluster command only takes tags as arguments,

whereas the node and instance commands first required the node and

instance name.

Tags can also be added from a file, via the ``--from=FILENAME``

argument. The file is expected to contain one tag per line.

Tags can also be remove via a syntax very similar to the add one::

  $ gnt-instance remove-tags %INSTANCE% %a% %b% %c%

And listed via::

  $ gnt-instance list-tags

  $ gnt-node list-tags

  $ gnt-cluster list-tags

Global tag search

+++++++++++++++++

It is also possible to execute a global search on the all tags defined

in the cluster configuration, via a cluster command::

  $ gnt-cluster search-tags %REGEXP%

The parameter expected is a regular expression (see

:manpage:`regex(7)`). This will return all tags that match the search,

together with the object they are defined in (the names being show in a

hierarchical kind of way)::

  $ gnt-cluster search-tags %o%

  /cluster foo

  /instances/instance1 owner:bar

Job operations

--------------

The various jobs submitted by the instance/node/cluster commands can be

examined, canceled and archived by various invocations of the

``gnt-job`` command.

First is the job list command::

  $ gnt-job list

  17771 success INSTANCE_QUERY_DATA

  17773 success CLUSTER_VERIFY_DISKS

  17775 success CLUSTER_REPAIR_DISK_SIZES

  17776 error   CLUSTER_RENAME(cluster.example.com)

  17780 success CLUSTER_REDIST_CONF

  17792 success INSTANCE_REBOOT(instance1.example.com)

More detailed information about a job can be found via the ``info``

command::

  $ gnt-job info %17776%

  Job ID: 17776

    Status: error

    Received:         2009-10-25 23:18:02.180569

    Processing start: 2009-10-25 23:18:02.200335 (delta 0.019766s)

    Processing end:   2009-10-25 23:18:02.279743 (delta 0.079408s)

    Total processing time: 0.099174 seconds

    Opcodes:

      OP_CLUSTER_RENAME

        Status: error

        Processing start: 2009-10-25 23:18:02.200335

        Processing end:   2009-10-25 23:18:02.252282

        Input fields:

          name: cluster.example.com

        Result:

          OpPrereqError

          [Neither the name nor the IP address of the cluster has changed]

        Execution log:

During the execution of a job, it's possible to follow the output of a

job, similar to the log that one get from the ``gnt-`` commands, via the

watch command::

  $ gnt-instance add --submit … %instance1%

  JobID: 17818

  $ gnt-job watch %17818%

  Output from job 17818 follows

  -----------------------------

  Mon Oct 26 00:22:48 2009  - INFO: Selected nodes for instance instance1 via iallocator dumb: node1, node2

  Mon Oct 26 00:22:49 2009 * creating instance disks...

  Mon Oct 26 00:22:52 2009 adding instance instance1 to cluster config

  Mon Oct 26 00:22:52 2009  - INFO: Waiting for instance instance1 to sync disks.

  …

  Mon Oct 26 00:23:03 2009 creating os for instance instance1 on node node1

  Mon Oct 26 00:23:03 2009 * running the instance OS create scripts...

  Mon Oct 26 00:23:13 2009 * starting instance...

  $

This is useful if you need to follow a job's progress from multiple

terminals.

A job that has not yet started to run can be canceled::

  $ gnt-job cancel %17810%

But not one that has already started execution::

  $ gnt-job cancel %17805%

  Job 17805 is no longer waiting in the queue

There are two queues for jobs: the *current* and the *archive*

queue. Jobs are initially submitted to the current queue, and they stay

in that queue until they have finished execution (either successfully or

not). At that point, they can be moved into the archive queue using e.g.

``gnt-job autoarchive all``. The ``ganeti-watcher`` script will do this

automatically 6 hours after a job is finished. The ``ganeti-cleaner``

script will then remove archived the jobs from the archive directory

after three weeks.

Note that ``gnt-job list`` only shows jobs in the current queue.

Archived jobs can be viewed using ``gnt-job info <id>``.

Special Ganeti deployments

--------------------------

Since Ganeti 2.4, it is possible to extend the Ganeti deployment with

two custom scenarios: Ganeti inside Ganeti and multi-site model.

Running Ganeti under Ganeti

+++++++++++++++++++++++++++

It is sometimes useful to be able to use a Ganeti instance as a Ganeti

node (part of another cluster, usually). One example scenario is two

small clusters, where we want to have an additional master candidate

that holds the cluster configuration and can be used for helping with

the master voting process.

However, these Ganeti instance should not host instances themselves, and

should not be considered in the normal capacity planning, evacuation

strategies, etc. In order to accomplish this, mark these nodes as

non-``vm_capable``::

  $ gnt-node modify --vm-capable=no %node3%

The vm_capable status can be listed as usual via ``gnt-node list``::

  $ gnt-node list -oname,vm_capable

  Node  VMCapable

  node1 Y

  node2 Y

  node3 N

When this flag is set, the cluster will not do any operations that

relate to instances on such nodes, e.g. hypervisor operations,

disk-related operations, etc. Basically they will just keep the ssconf

files, and if master candidates the full configuration.

Multi-site model

++++++++++++++++

If Ganeti is deployed in multi-site model, with each site being a node

group (so that instances are not relocated across the WAN by mistake),

it is conceivable that either the WAN latency is high or that some sites

have a lower reliability than others. In this case, it doesn't make

sense to replicate the job information across all sites (or even outside

of a “central” node group), so it should be possible to restrict which

nodes can become master candidates via the auto-promotion algorithm.

Ganeti 2.4 introduces for this purpose a new ``master_capable`` flag,

which (when unset) prevents nodes from being marked as master

candidates, either manually or automatically.

As usual, the node modify operation can change this flag::

  $ gnt-node modify --auto-promote --master-capable=no %node3%

  Fri Jan  7 06:23:07 2011  - INFO: Demoting from master candidate

  Fri Jan  7 06:23:08 2011  - INFO: Promoted nodes to master candidate role: node4

  Modified node node3

   - master_capable -> False

   - master_candidate -> False