[pve-docs.git] / ha-manager.adoc
1 [[chapter-ha-manager]]
2 ifdef::manvolnum[]
3 PVE({manvolnum})
4 ================
5 include::attributes.txt[]
8 ----
10 ha-manager - Proxmox VE HA Manager
13 --------
15 include::ha-manager.1-synopsis.adoc[]
18 -----------
19 endif::manvolnum[]
21 ifndef::manvolnum[]
22 High Availability
23 =================
24 include::attributes.txt[]
25 endif::manvolnum[]
28 Our modern society depends heavily on information provided by
29 computers over the network. Mobile devices amplified that dependency,
30 because people can access the network any time from anywhere. If you
31 provide such services, it is very important that they are available
32 most of the time.
34 We can mathematically define the availability as the ratio of (A) the
35 total time a service is capable of being used during a given interval
36 to (B) the length of the interval. It is normally expressed as a
37 percentage of uptime in a given year.
39 .Availability - Downtime per Year
40 [width="60%",cols="<d,d",options="header"]
41 |===========================================================
42 |Availability % |Downtime per year
43 |99 |3.65 days
44 |99.9 |8.76 hours
45 |99.99 |52.56 minutes
46 |99.999 |5.26 minutes
47 |99.9999 |31.5 seconds
48 |99.99999 |3.15 seconds
49 |===========================================================
51 There are several ways to increase availability. The most elegant
52 solution is to rewrite your software, so that you can run it on
53 several host at the same time. The software itself need to have a way
54 to detect errors and do failover. This is relatively easy if you just
55 want to serve read-only web pages. But in general this is complex, and
56 sometimes impossible because you cannot modify the software
57 yourself. The following solutions works without modifying the
58 software:
60 * Use reliable "server" components
62 NOTE: Computer components with same functionality can have varying
63 reliability numbers, depending on the component quality. Most vendors
64 sell components with higher reliability as "server" components -
65 usually at higher price.
67 * Eliminate single point of failure (redundant components)
69 - use an uninterruptible power supply (UPS)
70 - use redundant power supplies on the main boards
71 - use ECC-RAM
72 - use redundant network hardware
73 - use RAID for local storage
74 - use distributed, redundant storage for VM data
76 * Reduce downtime
78 - rapidly accessible administrators (24/7)
79 - availability of spare parts (other nodes in a {pve} cluster)
80 - automatic error detection ('ha-manager')
81 - automatic failover ('ha-manager')
83 Virtualization environments like {pve} makes it much easier to reach
84 high availability because they remove the "hardware" dependency. They
85 also support to setup and use redundant storage and network
86 devices. So if one host fail, you can simply start those services on
87 another host within your cluster.
89 Even better, {pve} provides a software stack called 'ha-manager',
90 which can do that automatically for you. It is able to automatically
91 detect errors and do automatic failover.
93 {pve} 'ha-manager' works like an "automated" administrator. First, you
94 configure what resources (VMs, containers, ...) it should
95 manage. 'ha-manager' then observes correct functionality, and handles
96 service failover to another node in case of errors. 'ha-manager' can
97 also handle normal user requests which may start, stop, relocate and
98 migrate a service.
100 But high availability comes at a price. High quality components are
101 more expensive, and making them redundant duplicates the costs at
102 least. Additional spare parts increase costs further. So you should
103 carefully calculate the benefits, and compare with those additional
104 costs.
106 TIP: Increasing availability from 99% to 99.9% is relatively
107 simply. But increasing availability from 99.9999% to 99.99999% is very
108 hard and costly. 'ha-manager' has typical error detection and failover
109 times of about 2 minutes, so you can get no more than 99.999%
110 availability.
112 Requirements
113 ------------
115 * at least three cluster nodes (to get reliable quorum)
117 * shared storage for VMs and containers
119 * hardware redundancy (everywhere)
121 * hardware watchdog - if not available we fall back to the
122 linux kernel software watchdog ('softdog')
124 * optional hardware fencing devices
127 Resources
128 ---------
130 We call the primary management unit handled by 'ha-manager' a
131 resource. A resource (also called "service") is uniquely
132 identified by a service ID (SID), which consists of the resource type
133 and an type specific ID, e.g.: 'vm:100'. That example would be a
134 resource of type 'vm' (virtual machine) with the ID 100.
136 For now we have two important resources types - virtual machines and
137 containers. One basic idea here is that we can bundle related software
138 into such VM or container, so there is no need to compose one big
139 service from other services, like it was done with 'rgmanager'. In
140 general, a HA enabled resource should not depend on other resources.
143 How It Works
144 ------------
146 This section provides an in detail description of the {PVE} HA-manager
147 internals. It describes how the CRM and the LRM work together.
149 To provide High Availability two daemons run on each node:
151 'pve-ha-lrm'::
153 The local resource manager (LRM), it controls the services running on
154 the local node.
155 It reads the requested states for its services from the current manager
156 status file and executes the respective commands.
158 'pve-ha-crm'::
160 The cluster resource manager (CRM), it controls the cluster wide
161 actions of the services, processes the LRM results and includes the state
162 machine which controls the state of each service.
164 .Locks in the LRM & CRM
165 [NOTE]
166 Locks are provided by our distributed configuration file system (pmxcfs).
167 They are used to guarantee that each LRM is active and working as a
168 LRM only executes actions when he has its lock we can mark a failed node
169 as fenced if we get its lock. This lets us then recover the failed HA services
170 securely without the failed (but maybe still running) LRM interfering.
171 This all gets supervised by the CRM which holds currently the manager master
172 lock.
174 Local Resource Manager
175 ~~~~~~~~~~~~~~~~~~~~~~
177 The local resource manager ('pve-ha-lrm') is started as a daemon on
178 boot and waits until the HA cluster is quorate and thus cluster wide
179 locks are working.
181 It can be in three states:
183 * *wait for agent lock*: the LRM waits for our exclusive lock. This is
184 also used as idle sate if no service is configured
185 * *active*: the LRM holds its exclusive lock and has services configured
186 * *lost agent lock*: the LRM lost its lock, this means a failure happened
187 and quorum was lost.
189 After the LRM gets in the active state it reads the manager status
190 file in '/etc/pve/ha/manager_status' and determines the commands it
191 has to execute for the services it owns.
192 For each command a worker gets started, this workers are running in
193 parallel and are limited to maximal 4 by default. This default setting
194 may be changed through the datacenter configuration key "max_worker".
195 When finished the worker process gets collected and its result saved for
196 the CRM.
198 .Maximal Concurrent Worker Adjustment Tips
199 [NOTE]
200 The default value of 4 maximal concurrent Workers may be unsuited for
201 a specific setup. For example may 4 live migrations happen at the same
202 time, which can lead to network congestions with slower networks and/or
203 big (memory wise) services. Ensure that also in the worst case no congestion
204 happens and lower the "max_worker" value if needed. In the contrary, if you
205 have a particularly powerful high end setup you may also want to increase it.
207 Each command requested by the CRM is uniquely identifiable by an UID, when
208 the worker finished its result will be processed and written in the LRM
209 status file '/etc/pve/nodes/<nodename>/lrm_status'. There the CRM may collect
210 it and let its state machine - respective the commands output - act on it.
212 The actions on each service between CRM and LRM are normally always synced.
213 This means that the CRM requests a state uniquely marked by an UID, the LRM
214 then executes this action *one time* and writes back the result, also
215 identifiable by the same UID. This is needed so that the LRM does not
216 executes an outdated command.
217 With the exception of the 'stop' and the 'error' command,
218 those two do not depend on the result produce and are executed
219 always in the case of the stopped state and once in the case of
220 the error state.
222 .Read the Logs
223 [NOTE]
224 The HA Stack logs every action it makes. This helps to understand what
225 and also why something happens in the cluster. Here its important to see
226 what both daemons, the LRM and the CRM, did. You may use
227 `journalctl -u pve-ha-lrm` on the node(s) where the service is and
228 the same command for the pve-ha-crm on the node which is the current master.
230 Cluster Resource Manager
231 ~~~~~~~~~~~~~~~~~~~~~~~~
233 The cluster resource manager ('pve-ha-crm') starts on each node and
234 waits there for the manager lock, which can only be held by one node
235 at a time. The node which successfully acquires the manager lock gets
236 promoted to the CRM master.
238 It can be in three states:
240 * *wait for agent lock*: the LRM waits for our exclusive lock. This is
241 also used as idle sate if no service is configured
242 * *active*: the LRM holds its exclusive lock and has services configured
243 * *lost agent lock*: the LRM lost its lock, this means a failure happened
244 and quorum was lost.
246 It main task is to manage the services which are configured to be highly
247 available and try to always enforce them to the wanted state, e.g.: a
248 enabled service will be started if its not running, if it crashes it will
249 be started again. Thus it dictates the LRM the actions it needs to execute.
251 When an node leaves the cluster quorum, its state changes to unknown.
252 If the current CRM then can secure the failed nodes lock, the services
253 will be 'stolen' and restarted on another node.
255 When a cluster member determines that it is no longer in the cluster
256 quorum, the LRM waits for a new quorum to form. As long as there is no
257 quorum the node cannot reset the watchdog. This will trigger a reboot
258 after the watchdog then times out, this happens after 60 seconds.
260 Configuration
261 -------------
263 The HA stack is well integrated in the Proxmox VE API2. So, for
264 example, HA can be configured via 'ha-manager' or the PVE web
265 interface, which both provide an easy to use tool.
267 The resource configuration file can be located at
268 '/etc/pve/ha/resources.cfg' and the group configuration file at
269 '/etc/pve/ha/groups.cfg'. Use the provided tools to make changes,
270 there shouldn't be any need to edit them manually.
272 Node Power Status
273 -----------------
275 If a node needs maintenance you should migrate and or relocate all
276 services which are required to run always on another node first.
277 After that you can stop the LRM and CRM services. But note that the
278 watchdog triggers if you stop it with active services.
280 Updates
281 ~~~~~~~
282 When updating the ha-manager you should do one node after the other, never
283 all at once. Further you have to ensure that no service located at the node
284 is in the error state, a node with erroneous service is not able to be upgraded
285 and if tried nonetheless it may even trigger a Node reset when doing so!
286 When dealing with erroneous services first check what happened to them, then
287 bring them in a secure state, after that disable or remove them from HA.
288 Only after that you may start upgrading a Nodes LRM and CRM.
290 Fencing
291 -------
293 What Is Fencing
294 ~~~~~~~~~~~~~~~
296 Fencing secures that on a node failure the dangerous node gets will be rendered
297 unable to do any damage and that no resource runs twice when it gets recovered
298 from the failed node.
300 Configure Hardware Watchdog
301 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
302 By default all watchdog modules are blocked for security reasons as they are
303 like a loaded gun if not correctly initialized.
304 If you have a hardware watchdog available remove its module from the blacklist
305 and restart 'the watchdog-mux' service.
308 Groups
309 ------
311 A group is a collection of cluster nodes which a service may be bound to.
313 Group Settings
314 ~~~~~~~~~~~~~~
316 nodes::
318 list of group node members
320 restricted::
322 resources bound to this group may only run on nodes defined by the
323 group. If no group node member is available the resource will be
324 placed in the stopped state.
326 nofailback::
328 the resource won't automatically fail back when a more preferred node
329 (re)joins the cluster.
332 Recovery Policy
333 ---------------
335 There are two service recover policy settings which can be configured
336 specific for each resource.
338 max_restart::
340 maximal number of tries to restart an failed service on the actual
341 node. The default is set to one.
343 max_relocate::
345 maximal number of tries to relocate the service to a different node.
346 A relocate only happens after the max_restart value is exceeded on the
347 actual node. The default is set to one.
349 NOTE: The relocate count state will only reset to zero when the
350 service had at least one successful start. That means if a service is
351 re-enabled without fixing the error only the restart policy gets
352 repeated.
354 Error Recovery
355 --------------
357 If after all tries the service state could not be recovered it gets
358 placed in an error state. In this state the service won't get touched
359 by the HA stack anymore. To recover from this state you should follow
360 these steps:
362 * bring the resource back into an safe and consistent state (e.g:
363 killing its process)
365 * disable the ha resource to place it in an stopped state
367 * fix the error which led to this failures
369 * *after* you fixed all errors you may enable the service again
372 Service Operations
373 ------------------
375 This are how the basic user-initiated service operations (via
376 'ha-manager') work.
378 enable::
380 the service will be started by the LRM if not already running.
382 disable::
384 the service will be stopped by the LRM if running.
386 migrate/relocate::
388 the service will be relocated (live) to another node.
390 remove::
392 the service will be removed from the HA managed resource list. Its
393 current state will not be touched.
395 start/stop::
397 start and stop commands can be issued to the resource specific tools
398 (like 'qm' or 'pct'), they will forward the request to the
399 'ha-manager' which then will execute the action and set the resulting
400 service state (enabled, disabled).
403 Service States
404 --------------
406 stopped::
408 Service is stopped (confirmed by LRM)
410 request_stop::
412 Service should be stopped. Waiting for confirmation from LRM.
414 started::
416 Service is active an LRM should start it ASAP if not already running.
418 fence::
420 Wait for node fencing (service node is not inside quorate cluster
421 partition).
423 freeze::
425 Do not touch the service state. We use this state while we reboot a
426 node, or when we restart the LRM daemon.
428 migrate::
430 Migrate service (live) to other node.
432 error::
434 Service disabled because of LRM errors. Needs manual intervention.
437 ifdef::manvolnum[]
438 include::pve-copyright.adoc[]
439 endif::manvolnum[]