[pve-docs.git] / ha-manager.adoc

[[chapter_ha_manager]]
ifdef::manvolnum[]
ha-manager(1)
=============
:pve-toplevel:

NAME
----

ha-manager - Proxmox VE HA Manager

SYNOPSIS
--------

include::ha-manager.1-synopsis.adoc[]

DESCRIPTION
-----------
endif::manvolnum[]
ifndef::manvolnum[]
High Availability
=================
:pve-toplevel:
endif::manvolnum[]

Our modern society depends heavily on information provided by
computers over the network. Mobile devices amplified that dependency,
because people can access the network any time from anywhere. If you
provide such services, it is very important that they are available
most of the time.

We can mathematically define the availability as the ratio of (A), the
total time a service is capable of being used during a given interval
to (B), the length of the interval. It is normally expressed as a
percentage of uptime in a given year.

.Availability - Downtime per Year
[width="60%",cols="<d,d",options="header"]
|===========================================================
|Availability % |Downtime per year
|99             |3.65 days
|99.9 		|8.76 hours
|99.99  	|52.56 minutes
|99.999  	|5.26 minutes
|99.9999  	|31.5 seconds
|99.99999 	|3.15 seconds
|===========================================================

There are several ways to increase availability. The most elegant
solution is to rewrite your software, so that you can run it on
several hosts at the same time. The software itself needs to have a way
to detect errors and do failover. If you only want to serve read-only
web pages, then this is relatively simple. However, this is generally complex
and sometimes impossible, because you cannot modify the software yourself. The
following solutions works without modifying the software:

* Use reliable ``server'' components
+
NOTE: Computer components with the same functionality can have varying
reliability numbers, depending on the component quality. Most vendors
sell components with higher reliability as ``server'' components -
usually at higher price.

* Eliminate single point of failure (redundant components)
** use an uninterruptible power supply (UPS)
** use redundant power supplies on the main boards
** use ECC-RAM
** use redundant network hardware
** use RAID for local storage
** use distributed, redundant storage for VM data

* Reduce downtime
** rapidly accessible administrators (24/7)
** availability of spare parts (other nodes in a {pve} cluster)
** automatic error detection (provided by `ha-manager`)
** automatic failover (provided by `ha-manager`)

Virtualization environments like {pve} make it much easier to reach
high availability because they remove the ``hardware'' dependency. They
also support the setup and use of redundant storage and network
devices, so if one host fails, you can simply start those services on
another host within your cluster.

Better still, {pve} provides a software stack called `ha-manager`,
which can do that automatically for you. It is able to automatically
detect errors and do automatic failover.

{pve} `ha-manager` works like an ``automated'' administrator. First, you
configure what resources (VMs, containers, ...) it should
manage. Then, `ha-manager` observes the correct functionality, and handles
service failover to another node in case of errors. `ha-manager` can
also handle normal user requests which may start, stop, relocate and
migrate a service.

But high availability comes at a price. High quality components are
more expensive, and making them redundant doubles the costs at
least. Additional spare parts increase costs further. So you should
carefully calculate the benefits, and compare with those additional
costs.

TIP: Increasing availability from 99% to 99.9% is relatively
simple. But increasing availability from 99.9999% to 99.99999% is very
hard and costly. `ha-manager` has typical error detection and failover
times of about 2 minutes, so you can get no more than 99.999%
availability.


Requirements
------------

You must meet the following requirements before you start with HA:

* at least three cluster nodes (to get reliable quorum)

* shared storage for VMs and containers

* hardware redundancy (everywhere)

* use reliable “server” components

* hardware watchdog - if not available we fall back to the
  linux kernel software watchdog (`softdog`)

* optional hardware fencing devices


[[ha_manager_resources]]
Resources
---------

We call the primary management unit handled by `ha-manager` a
resource. A resource (also called ``service'') is uniquely
identified by a service ID (SID), which consists of the resource type
and a type specific ID, for example `vm:100`. That example would be a
resource of type `vm` (virtual machine) with the ID 100.

For now we have two important resources types - virtual machines and
containers. One basic idea here is that we can bundle related software
into such a VM or container, so there is no need to compose one big
service from other services, as was done with `rgmanager`. In
general, a HA managed resource should not depend on other resources.


Management Tasks
----------------

This section provides a short overview of common management tasks. The
first step is to enable HA for a resource. This is done by adding the
resource to the HA resource configuration. You can do this using the
GUI, or simply use the command line tool, for example:

----
# ha-manager add vm:100
----

The HA stack now tries to start the resources and keep them
running. Please note that you can configure the ``requested''
resources state. For example you may want the HA stack to stop the
resource:

----
# ha-manager set vm:100 --state stopped
----

and start it again later:

----
# ha-manager set vm:100 --state started
----

You can also use the normal VM and container management commands. They
automatically forward the commands to the HA stack, so

----
# qm start 100
----

simply sets the requested state to `started`. The same applies to `qm
stop`, which sets the requested state to `stopped`.

NOTE: The HA stack works fully asynchronous and needs to communicate
with other cluster members. Therefore, it takes some seconds until you see
the result of such actions.

To view the current HA resource configuration use:

----
# ha-manager config
vm:100
	state stopped
----

And you can view the actual HA manager and resource state with:

----
# ha-manager status
quorum OK
master node1 (active, Wed Nov 23 11:07:23 2016)
lrm elsa (active, Wed Nov 23 11:07:19 2016)
service vm:100 (node1, started)
----

You can also initiate resource migration to other nodes:

----
# ha-manager migrate vm:100 node2
----

This uses online migration and tries to keep the VM running. Online
migration needs to transfer all used memory over the network, so it is
sometimes faster to stop the VM, then restart it on the new node. This can be
done using the `relocate` command:

----
# ha-manager relocate vm:100 node2
----

Finally, you can remove the resource from the HA configuration using
the following command:

----
# ha-manager remove vm:100
----

NOTE: This does not start or stop the resource.

But all HA related tasks can be done in the GUI, so there is no need to
use the command line at all.


How It Works
------------

This section provides a detailed description of the {PVE} HA manager
internals. It describes all involved daemons and how they work
together. To provide HA, two daemons run on each node:

`pve-ha-lrm`::

The local resource manager (LRM), which controls the services running on
the local node. It reads the requested states for its services from
the current manager status file and executes the respective commands.

`pve-ha-crm`::

The cluster resource manager (CRM), which makes the cluster-wide
decisions. It sends commands to the LRM, processes the results,
and moves resources to other nodes if something fails. The CRM also
handles node fencing.


.Locks in the LRM & CRM
[NOTE]
Locks are provided by our distributed configuration file system (pmxcfs).
They are used to guarantee that each LRM is active once and working. As an
LRM only executes actions when it holds its lock, we can mark a failed node
as fenced if we can acquire its lock. This then lets us recover any failed
HA services securely without any interference from the now unknown failed node.
This all gets supervised by the CRM which currently holds the manager master
lock.


Service States
~~~~~~~~~~~~~~

The CRM uses a service state enumeration to record the current service
state. This state is displayed on the GUI and can be queried using
the `ha-manager` command line tool:

----
# ha-manager status
quorum OK
master elsa (active, Mon Nov 21 07:23:29 2016)
lrm elsa (active, Mon Nov 21 07:23:22 2016)
service ct:100 (elsa, stopped)
service ct:102 (elsa, started)
service vm:501 (elsa, started)
----

Here is the list of possible states:

stopped::

Service is stopped (confirmed by LRM). If the LRM detects a stopped
service is still running, it will stop it again.

request_stop::

Service should be stopped. The CRM waits for confirmation from the
LRM.

stopping::

Pending stop request. But the CRM did not get the request so far.

started::

Service is active an LRM should start it ASAP if not already running.
If the Service fails and is detected to be not running the LRM
restarts it
(see xref:ha_manager_start_failure_policy[Start Failure Policy]).

starting::

Pending start request. But the CRM has not got any confirmation from the
LRM that the service is running.

fence::

Wait for node fencing as the service node is not inside the quorate cluster
partition (see xref:ha_manager_fencing[Fencing]).
As soon as node gets fenced successfully the service will be placed into the
recovery state.

recovery::

Wait for recovery of the service. The HA manager tries to find a new node where
the service can run on. This search depends not only on the list of online and
quorate nodes, but also if the service is a group member and how such a group
is limited.
As soon as a new available node is found, the service will be moved there and
initially placed into stopped state. If it's configured to run the new node
will do so.

freeze::

Do not touch the service state. We use this state while we reboot a
node, or when we restart the LRM daemon
(see xref:ha_manager_package_updates[Package Updates]).

ignored::

Act as if the service were not managed by HA at all.
Useful, when full control over the service is desired temporarily, without
removing it from the HA configuration.

migrate::

Migrate service (live) to other node.

error::

Service is disabled because of LRM errors. Needs manual intervention
(see xref:ha_manager_error_recovery[Error Recovery]).

queued::

Service is newly added, and the CRM has not seen it so far.

disabled::

Service is stopped and marked as `disabled`


Local Resource Manager
~~~~~~~~~~~~~~~~~~~~~~

The local resource manager (`pve-ha-lrm`) is started as a daemon on
boot and waits until the HA cluster is quorate and thus cluster-wide
locks are working.

It can be in three states:

wait for agent lock::

The LRM waits for our exclusive lock. This is also used as idle state if no
service is configured.

active::

The LRM holds its exclusive lock and has services configured.

lost agent lock::

The LRM lost its lock, this means a failure happened and quorum was lost.

After the LRM gets in the active state it reads the manager status
file in `/etc/pve/ha/manager_status` and determines the commands it
has to execute for the services it owns.
For each command a worker gets started, these workers are running in
parallel and are limited to at most 4 by default. This default setting
may be changed through the datacenter configuration key `max_worker`.
When finished the worker process gets collected and its result saved for
the CRM.

.Maximum Concurrent Worker Adjustment Tips
[NOTE]
The default value of at most 4 concurrent workers may be unsuited for
a specific setup. For example, 4 live migrations may occur at the same
time, which can lead to network congestions with slower networks and/or
big (memory wise) services. Also, ensure that in the worst case, congestion is
at a minimum, even if this means lowering the `max_worker` value. On the
contrary, if you have a particularly powerful, high-end setup you may also want
to increase it.

Each command requested by the CRM is uniquely identifiable by a UID. When
the worker finishes, its result will be processed and written in the LRM
status file `/etc/pve/nodes/<nodename>/lrm_status`. There the CRM may collect
it and let its state machine - respective to the commands output - act on it.

The actions on each service between CRM and LRM are normally always synced.
This means that the CRM requests a state uniquely marked by a UID, the LRM
then executes this action *one time* and writes back the result, which is also
identifiable by the same UID. This is needed so that the LRM does not
execute an outdated command.
The only exceptions to this behaviour are the `stop` and `error` commands;
these two do not depend on the result produced and are executed
always in the case of the stopped state and once in the case of
the error state.

.Read the Logs
[NOTE]
The HA Stack logs every action it makes. This helps to understand what
and also why something happens in the cluster. Here its important to see
what both daemons, the LRM and the CRM, did. You may use
`journalctl -u pve-ha-lrm` on the node(s) where the service is and
the same command for the pve-ha-crm on the node which is the current master.

Cluster Resource Manager
~~~~~~~~~~~~~~~~~~~~~~~~

The cluster resource manager (`pve-ha-crm`) starts on each node and
waits there for the manager lock, which can only be held by one node
at a time.  The node which successfully acquires the manager lock gets
promoted to the CRM master.

It can be in three states:

wait for agent lock::

The CRM waits for our exclusive lock. This is also used as idle state if no
service is configured

active::

The CRM holds its exclusive lock and has services configured

lost agent lock::

The CRM lost its lock, this means a failure happened and quorum was lost.

Its main task is to manage the services which are configured to be highly
available and try to always enforce the requested state. For example, a
service with the requested state 'started' will be started if its not
already running. If it crashes it will be automatically started again.
Thus the CRM dictates the actions the LRM needs to execute.

When a node leaves the cluster quorum, its state changes to unknown.
If the current CRM can then secure the failed node's lock, the services
will be 'stolen' and restarted on another node.

When a cluster member determines that it is no longer in the cluster
quorum, the LRM waits for a new quorum to form. As long as there is no
quorum the node cannot reset the watchdog. This will trigger a reboot
after the watchdog times out (this happens after 60 seconds).


HA Simulator
------------

[thumbnail="screenshot/gui-ha-manager-status.png"]

By using the HA simulator you can test and learn all functionalities of the
Proxmox VE HA solutions.

By default, the simulator allows you to watch and test the behaviour of a
real-world 3 node cluster with 6 VMs. You can also add or remove additional VMs
or Container.

You do not have to setup or configure a real cluster, the HA simulator runs out
of the box.

Install with apt:

----
apt install pve-ha-simulator
----

You can even install the package on any Debian-based system without any
other Proxmox VE packages.  For that you will need to download the package and
copy it to the system you want to run it on for installation.  When you install
the package with apt from the local file system it will also resolve the
required dependencies for you.


To start the simulator on a remote machine you must have an X11 redirection to
your current system.

If you are on a Linux machine you can use:

----
ssh root@<IPofPVE> -Y
----

On Windows it works with https://mobaxterm.mobatek.net/[mobaxterm].

After connecting to an existing {pve} with the simulator installed or
installing it on your local Debian-based system manually, you can try it out as
follows.

First you need to create a working directory where the simulator saves its
current state and writes its default config:

----
mkdir working
----

Then, simply pass the created directory as a parameter to 'pve-ha-simulator':

----
pve-ha-simulator working/
----

You can then start, stop, migrate the simulated HA services, or even check out
what happens on a node failure.

Configuration
-------------

The HA stack is well integrated into the {pve} API. So, for example,
HA can be configured via the `ha-manager` command line interface, or
the {pve} web interface - both interfaces provide an easy way to
manage HA. Automation tools can use the API directly.

All HA configuration files are within `/etc/pve/ha/`, so they get
automatically distributed to the cluster nodes, and all nodes share
the same HA configuration.


[[ha_manager_resource_config]]
Resources
~~~~~~~~~

[thumbnail="screenshot/gui-ha-manager-status.png"]


The resource configuration file `/etc/pve/ha/resources.cfg` stores
the list of resources managed by `ha-manager`. A resource configuration
inside that list looks like this:

----
<type>: <name>
	<property> <value>
	...
----

It starts with a resource type followed by a resource specific name,
separated with colon. Together this forms the HA resource ID, which is
used by all `ha-manager` commands to uniquely identify a resource
(example: `vm:100` or `ct:101`). The next lines contain additional
properties:

include::ha-resources-opts.adoc[]

Here is a real world example with one VM and one container. As you see,
the syntax of those files is really simple, so it is even possible to
read or edit those files using your favorite editor:

.Configuration Example (`/etc/pve/ha/resources.cfg`)
----
vm: 501
    state started
    max_relocate 2

ct: 102
    # Note: use default settings for everything
----

[thumbnail="screenshot/gui-ha-manager-add-resource.png"]

The above config was generated using the `ha-manager` command line tool:

----
# ha-manager add vm:501 --state started --max_relocate 2
# ha-manager add ct:102
----


[[ha_manager_groups]]
Groups
~~~~~~

[thumbnail="screenshot/gui-ha-manager-groups-view.png"]

The HA group configuration file `/etc/pve/ha/groups.cfg` is used to
define groups of cluster nodes. A resource can be restricted to run
only on the members of such group. A group configuration look like
this:

----
group: <group>
       nodes <node_list>
       <property> <value>
       ...
----

include::ha-groups-opts.adoc[]

[thumbnail="screenshot/gui-ha-manager-add-group.png"]

A common requirement is that a resource should run on a specific
node. Usually the resource is able to run on other nodes, so you can define
an unrestricted group with a single member:

----
# ha-manager groupadd prefer_node1 --nodes node1
----

For bigger clusters, it makes sense to define a more detailed failover
behavior. For example, you may want to run a set of services on
`node1` if possible. If `node1` is not available, you want to run them
equally split on `node2` and `node3`. If those nodes also fail, the
services should run on `node4`. To achieve this you could set the node
list to:

----
# ha-manager groupadd mygroup1 -nodes "node1:2,node2:1,node3:1,node4"
----

Another use case is if a resource uses other resources only available
on specific nodes, lets say `node1` and `node2`. We need to make sure
that HA manager does not use other nodes, so we need to create a
restricted group with said nodes:

----
# ha-manager groupadd mygroup2 -nodes "node1,node2" -restricted
----

The above commands created the following group configuration file:

.Configuration Example (`/etc/pve/ha/groups.cfg`)
----
group: prefer_node1
       nodes node1

group: mygroup1
       nodes node2:1,node4,node1:2,node3:1

group: mygroup2
       nodes node2,node1
       restricted 1
----


The `nofailback` options is mostly useful to avoid unwanted resource
movements during administration tasks. For example, if you need to
migrate a service to a node which doesn't have the highest priority in the
group, you need to tell the HA manager not to instantly move this service
back by setting the `nofailback` option.

Another scenario is when a service was fenced and it got recovered to
another node. The admin tries to repair the fenced node and brings it
up online again to investigate the cause of failure and check if it runs
stably again. Setting the `nofailback` flag prevents the recovered services from
moving straight back to the fenced node.


[[ha_manager_fencing]]
Fencing
-------

On node failures, fencing ensures that the erroneous node is
guaranteed to be offline. This is required to make sure that no
resource runs twice when it gets recovered on another node. This is a
really important task, because without this, it would not be possible to
recover a resource on another node.

If a node did not get fenced, it would be in an unknown state where
it may have still access to shared resources. This is really
dangerous! Imagine that every network but the storage one broke. Now,
while not reachable from the public network, the VM still runs and
writes to the shared storage.

If we then simply start up this VM on another node, we would get a
dangerous race condition, because we write from both nodes. Such
conditions can destroy all VM data and the whole VM could be rendered
unusable. The recovery could also fail if the storage protects against
multiple mounts.


How {pve} Fences
~~~~~~~~~~~~~~~~

There are different methods to fence a node, for example, fence
devices which cut off the power from the node or disable their
communication completely. Those are often quite expensive and bring
additional critical components into a system, because if they fail you
cannot recover any service.

We thus wanted to integrate a simpler fencing method, which does not
require additional external hardware. This can be done using
watchdog timers.

.Possible Fencing Methods
- external power switches
- isolate nodes by disabling complete network traffic on the switch
- self fencing using watchdog timers

Watchdog timers have been widely used in critical and dependable systems
since the beginning of microcontrollers. They are often simple, independent
integrated circuits which are used to detect and recover from computer malfunctions.

During normal operation, `ha-manager` regularly resets the watchdog
timer to prevent it from elapsing. If, due to a hardware fault or
program error, the computer fails to reset the watchdog, the timer
will elapse and trigger a reset of the whole server (reboot).

Recent server motherboards often include such hardware watchdogs, but
these need to be configured. If no watchdog is available or
configured, we fall back to the Linux Kernel 'softdog'. While still
reliable, it is not independent of the servers hardware, and thus has
a lower reliability than a hardware watchdog.


Configure Hardware Watchdog
~~~~~~~~~~~~~~~~~~~~~~~~~~~

By default, all hardware watchdog modules are blocked for security
reasons. They are like a loaded gun if not correctly initialized. To
enable a hardware watchdog, you need to specify the module to load in
'/etc/default/pve-ha-manager', for example:

----
# select watchdog module (default is softdog)
WATCHDOG_MODULE=iTCO_wdt
----

This configuration is read by the 'watchdog-mux' service, which loads
the specified module at startup.


Recover Fenced Services
~~~~~~~~~~~~~~~~~~~~~~~

After a node failed and its fencing was successful, the CRM tries to
move services from the failed node to nodes which are still online.

The selection of nodes, on which those services gets recovered, is
influenced by the resource `group` settings, the list of currently active
nodes, and their respective active service count.

The CRM first builds a set out of the intersection between user selected
nodes (from `group` setting) and available nodes. It then choose the
subset of nodes with the highest priority, and finally select the node
with the lowest active service count. This minimizes the possibility
of an overloaded node.

CAUTION: On node failure, the CRM distributes services to the
remaining nodes. This increases the service count on those nodes, and
can lead to high load, especially on small clusters. Please design
your cluster so that it can handle such worst case scenarios.


[[ha_manager_start_failure_policy]]
Start Failure Policy
---------------------

The start failure policy comes into effect if a service failed to start on a
node one or more times. It can be used to configure how often a restart
should be triggered on the same node and how often a service should be
relocated, so that it has an attempt to be started on another node.
The aim of this policy is to circumvent temporary unavailability of shared
resources on a specific node. For example, if a shared storage isn't available
on a quorate node anymore, for instance due to network problems, but is still
available on other nodes, the relocate policy allows the service to start
nonetheless.

There are two service start recover policy settings which can be configured
specific for each resource.

max_restart::

Maximum number of attempts to restart a failed service on the actual
node.  The default is set to one.

max_relocate::

Maximum number of attempts to relocate the service to a different node.
A relocate only happens after the max_restart value is exceeded on the
actual node. The default is set to one.

NOTE: The relocate count state will only reset to zero when the
service had at least one successful start. That means if a service is
re-started without fixing the error only the restart policy gets
repeated.


[[ha_manager_error_recovery]]
Error Recovery
--------------

If, after all attempts, the service state could not be recovered, it gets
placed in an error state. In this state, the service won't get touched
by the HA stack anymore. The only way out is disabling a service:

----
# ha-manager set vm:100 --state disabled
----

This can also be done in the web interface.

To recover from the error state you should do the following:

* bring the resource back into a safe and consistent state (e.g.:
kill its process if the service could not be stopped)

* disable the resource to remove the error flag

* fix the error which led to this failures

* *after* you fixed all errors you may request that the service starts again


[[ha_manager_package_updates]]
Package Updates
---------------

When updating the ha-manager, you should do one node after the other, never
all at once for various reasons. First, while we test our software
thoroughly, a bug affecting your specific setup cannot totally be ruled out.
Updating one node after the other and checking the functionality of each node
after finishing the update helps to recover from eventual problems, while
updating all at once could result in a broken cluster and is generally not
good practice.

Also, the {pve} HA stack uses a request acknowledge protocol to perform
actions between the cluster and the local resource manager. For restarting,
the LRM makes a request to the CRM to freeze all its services. This prevents
them from getting touched by the Cluster during the short time the LRM is restarting.
After that, the LRM may safely close the watchdog during a restart.
Such a restart happens normally during a package update and, as already stated,
an active master CRM is needed to acknowledge the requests from the LRM. If
this is not the case the update process can take too long which, in the worst
case, may result in a reset triggered by the watchdog.


Node Maintenance
----------------

It is sometimes necessary to shutdown or reboot a node to do maintenance tasks,
such as to replace hardware, or simply to install a new kernel image. This is
also true when using the HA stack. The behaviour of the HA stack during a
shutdown can be configured.

[[ha_manager_shutdown_policy]]
Shutdown Policy
~~~~~~~~~~~~~~~

Below you will find a description of the different HA policies for a node
shutdown. Currently 'Conditional' is the default due to backward compatibility.
Some users may find that 'Migrate' behaves more as expected.

Migrate
^^^^^^^

Once the Local Resource manager (LRM) gets a shutdown request and this policy
is enabled, it will mark itself as unavailable for the current HA manager.
This triggers a migration of all HA Services currently located on this node.
The LRM will try to delay the shutdown process, until all running services get
moved away. But, this expects that the running services *can* be migrated to
another node. In other words, the service must not be locally bound, for example
by using hardware passthrough. As non-group member nodes are considered as
runnable target if no group member is available, this policy can still be used
when making use of HA groups with only some nodes selected. But, marking a group
as 'restricted' tells the HA manager that the service cannot run outside of the
chosen set of nodes. If all of those nodes are unavailable, the shutdown will
hang until you manually intervene. Once the shut down node comes back online
again, the previously displaced services will be moved back, if they were not
already manually migrated in-between.

NOTE: The watchdog is still active during the migration process on shutdown.
If the node loses quorum it will be fenced and the services will be recovered.

If you start a (previously stopped) service on a node which is currently being
maintained, the node needs to be fenced to ensure that the service can be moved
and started on another available node.

Failover
^^^^^^^^

This mode ensures that all services get stopped, but that they will also be
recovered, if the current node is not online soon. It can be useful when doing
maintenance on a cluster scale, where live-migrating VMs may not be possible if
too many nodes are powered off at a time, but you still want to ensure HA
services get recovered and started again as soon as possible.

Freeze
^^^^^^

This mode ensures that all services get stopped and frozen, so that they won't
get recovered until the current node is online again.

Conditional
^^^^^^^^^^^

The 'Conditional' shutdown policy automatically detects if a shutdown or a
reboot is requested, and changes behaviour accordingly.

.Shutdown

A shutdown ('poweroff') is usually done if it is planned for the node to stay
down for some time. The LRM stops all managed services in this case. This means
that other nodes will take over those services afterwards.

NOTE: Recent hardware has large amounts of memory (RAM). So we stop all
resources, then restart them to avoid online migration of all that RAM. If you
want to use online migration, you need to invoke that manually before you
shutdown the node.


.Reboot

Node reboots are initiated with the 'reboot' command. This is usually done
after installing a new kernel. Please note that this is different from
``shutdown'', because the node immediately starts again.

The LRM tells the CRM that it wants to restart, and waits until the CRM puts
all resources into the `freeze` state (same mechanism is used for
xref:ha_manager_package_updates[Package Updates]). This prevents those resources
from being moved to other nodes. Instead, the CRM starts the resources after the
reboot on the same node.


Manual Resource Movement
^^^^^^^^^^^^^^^^^^^^^^^^

Last but not least, you can also manually move resources to other nodes, before
you shutdown or restart a node. The advantage is that you have full control,
and you can decide if you want to use online migration or not.

NOTE: Please do not 'kill' services like `pve-ha-crm`, `pve-ha-lrm` or
`watchdog-mux`. They manage and use the watchdog, so this can result in an
immediate node reboot or even reset.


[[ha_manager_crs]]
Cluster Resource Scheduling
---------------------------

The scheduler mode controls how HA selects nodes for the recovery of a service
as well as for migrations that are triggered by a shutdown policy. The default
mode is `basic`, you can change it in `datacenter.cfg`:

----
crs: ha=static
----

The change will be in effect starting with the next manager round (after a few
seconds).

For each service that needs to be recovered or migrated, the scheduler
iteratively chooses the best node among the nodes with the highest priority in
the service's group.

NOTE: There are plans to add modes for (static and dynamic) load-balancing in
the future.

Basic
~~~~~

The number of active HA services on each node is used to choose a recovery node.

Static
~~~~~~

IMPORTANT: The static mode is still a technology preview.

Static usage information from HA services on each node is used to choose a
recovery node.

For this selection, each node in turn is considered as if the service was
already running on it, using CPU and memory usage from the associated guest
configuration. Then for each such alternative, CPU and memory usage of all nodes
are considered, with memory being weighted much more, because it's a truly
limited resource. For both, CPU and memory, highest usage among nodes (weighted
more, as ideally no node should be overcommitted) and average usage of all nodes
(to still be able to distinguish in case there already is a more highly
committed node) are considered.

IMPORTANT: The more services the more possible combinations there are, so it's
currently not recommended to use it if you have thousands of HA managed
services.


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