5b51d5c4c56d21150ffae7e4d0bea76136b3aba0
[pve-docs.git] / ha-manager.adoc
1 [[chapter-ha-manager]]
2 ifdef::manvolnum[]
3 PVE({manvolnum})
4 ================
5 include::attributes.txt[]
6
7 NAME
8 ----
9
10 ha-manager - Proxmox VE HA Manager
11
12 SYNOPSYS
13 --------
14
15 include::ha-manager.1-synopsis.adoc[]
16
17 DESCRIPTION
18 -----------
19 endif::manvolnum[]
20
21 ifndef::manvolnum[]
22 High Availability
23 =================
24 include::attributes.txt[]
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26
27
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.
33
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.
38
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 |===========================================================
50
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 erors 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:
59
60 * Use reliable "server" components
61
62 NOTE: Computer components with same functionality can have varying
63 reliability numbers, depending on the component quality. Most verdors
64 sell components with higher reliability as "server" components -
65 usually at higher price.
66
67 * Eliminate single point of failure (redundant components)
68
69 - use an uniteruptable 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
75
76 * Reduce downtime
77
78 - rapidly accessible adminstrators (24/7)
79 - availability of spare parts (other nodes is a {pve} cluster)
80 - automatic error detection ('ha-manager')
81 - automatic failover ('ha-manager')
82
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.
88
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.
92
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.
99
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.
105
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.
111
112 Requirements
113 ------------
114
115 * at least three cluster nodes (to get reliable quorum)
116
117 * shared storage for VMs and containers
118
119 * hardware redundancy (everywhere)
120
121 * hardware watchdog - if not available we fall back to the
122 linux kernel software watchdog ('softdog')
123
124 * optional hardware fencing devices
125
126
127 Resources
128 ---------
129
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.
135
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.
141
142
143 How It Works
144 ------------
145
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.
148
149 To provide High Availability two daemons run on each node:
150
151 'pve-ha-lrm'::
152
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.
157
158 'pve-ha-crm'::
159
160 The cluster resource manager (CRM), it controls the cluster wide
161 actions of the services, processes the LRM result includes the state
162 machine which controls the state of each service.
163
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.
173
174 Local Resource Manager
175 ~~~~~~~~~~~~~~~~~~~~~~
176
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.
180
181 It can be in three states:
182
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.
188
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 service 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
196 .Maximal Concurrent Worker Adjustment Tips
197 [NOTE]
198 The default value of 4 maximal concurrent Workers may be unsuited for
199 a specific setup. For example may 4 live migrations happen at the same
200 time, which can lead to network congestions with slower networks and/or
201 big (memory wise) services. Ensure that also in the worst case no congestion
202 happens and lower the "max_worker" value if needed. In the contrary, if you
203 have a particularly powerful high end setup you may also want to increase it.
204
205 Each command requested by the CRM is uniquely identifiable by an UID, when
206 the worker finished its result will be processed and written in the LRM
207 status file '/etc/pve/nodes/<nodename>/lrm_status'. There the CRM may collect
208 it and let its state machine - respective the commands output - act on it.
209
210 The actions on each service between CRM and LRM are normally always synced.
211 This means that the CRM requests a state uniquely marked by an UID, the LRM
212 then executes this action *one time* and writes back the result, also
213 identifiable by the same UID. This is needed so that the LRM does not
214 executes an outdated command.
215 With the exception of the 'stop' and the 'error' command,
216 those two do not depend on the result produce and are executed
217 always in the case of the stopped state and once in the case of
218 the error state.
219
220 .Read the Logs
221 [NOTE]
222 The HA Stack logs every action it makes. This helps to understand what
223 and also why something happens in the cluster. Here its important to see
224 what both daemons, the LRM and the CRM, did. You may use
225 `journalctl -u pve-ha-lrm` on the node(s) where the service is and
226 the same command for the pve-ha-crm on the node which is the current master.
227
228 Cluster Resource Manager
229 ~~~~~~~~~~~~~~~~~~~~~~~~
230
231 The cluster resource manager ('pve-ha-crm') starts on each node and
232 waits there for the manager lock, which can only be held by one node
233 at a time. The node which successfully acquires the manager lock gets
234 promoted to the CRM master.
235
236 It can be in three states: TODO
237
238 * *wait for agent lock*: the LRM waits for our exclusive lock. This is
239 also used as idle sate if no service is configured
240 * *active*: the LRM holds its exclusive lock and has services configured
241 * *lost agent lock*: the LRM lost its lock, this means a failure happened
242 and quorum was lost.
243
244 It main task is to manage the services which are configured to be highly
245 available and try to get always bring them in the wanted state, e.g.: a
246 enabled service will be started if its not running, if it crashes it will
247 be started again. Thus it dictates the LRM the wanted actions.
248
249 When an node leaves the cluster quorum, its state changes to unknown.
250 If the current CRM then can secure the failed nodes lock, the services
251 will be 'stolen' and restarted on another node.
252
253 When a cluster member determines that it is no longer in the cluster
254 quorum, the LRM waits for a new quorum to form. As long as there is no
255 quorum the node cannot reset the watchdog. This will trigger a reboot
256 after 60 seconds.
257
258 Configuration
259 -------------
260
261 The HA stack is well integrated int the Proxmox VE API2. So, for
262 example, HA can be configured via 'ha-manager' or the PVE web
263 interface, which both provide an easy to use tool.
264
265 The resource configuration file can be located at
266 '/etc/pve/ha/resources.cfg' and the group configuration file at
267 '/etc/pve/ha/groups.cfg'. Use the provided tools to make changes,
268 there shouldn't be any need to edit them manually.
269
270 Node Power Status
271 -----------------
272
273 If a node needs maintenance you should migrate and or relocate all
274 services which are required to run always on another node first.
275 After that you can stop the LRM and CRM services. But note that the
276 watchdog triggers if you stop it with active services.
277
278 Fencing
279 -------
280
281 What Is Fencing
282 ~~~~~~~~~~~~~~~
283
284 Fencing secures that on a node failure the dangerous node gets will be rendered
285 unable to do any damage and that no resource runs twice when it gets recovered
286 from the failed node.
287
288 Configure Hardware Watchdog
289 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
290 By default all watchdog modules are blocked for security reasons as they are
291 like a loaded gun if not correctly initialized.
292 If you have a hardware watchdog available remove its module from the blacklist
293 and restart 'the watchdog-mux' service.
294
295
296 Groups
297 ------
298
299 A group is a collection of cluster nodes which a service may be bound to.
300
301 Group Settings
302 ~~~~~~~~~~~~~~
303
304 nodes::
305
306 list of group node members
307
308 restricted::
309
310 resources bound to this group may only run on nodes defined by the
311 group. If no group node member is available the resource will be
312 placed in the stopped state.
313
314 nofailback::
315
316 the resource won't automatically fail back when a more preferred node
317 (re)joins the cluster.
318
319
320 Recovery Policy
321 ---------------
322
323 There are two service recover policy settings which can be configured
324 specific for each resource.
325
326 max_restart::
327
328 maximal number of tries to restart an failed service on the actual
329 node. The default is set to one.
330
331 max_relocate::
332
333 maximal number of tries to relocate the service to a different node.
334 A relocate only happens after the max_restart value is exceeded on the
335 actual node. The default is set to one.
336
337 NOTE: The relocate count state will only reset to zero when the
338 service had at least one successful start. That means if a service is
339 re-enabled without fixing the error only the restart policy gets
340 repeated.
341
342 Error Recovery
343 --------------
344
345 If after all tries the service state could not be recovered it gets
346 placed in an error state. In this state the service won't get touched
347 by the HA stack anymore. To recover from this state you should follow
348 these steps:
349
350 * bring the resource back into an safe and consistent state (e.g:
351 killing its process)
352
353 * disable the ha resource to place it in an stopped state
354
355 * fix the error which led to this failures
356
357 * *after* you fixed all errors you may enable the service again
358
359
360 Service Operations
361 ------------------
362
363 This are how the basic user-initiated service operations (via
364 'ha-manager') work.
365
366 enable::
367
368 the service will be started by the LRM if not already running.
369
370 disable::
371
372 the service will be stopped by the LRM if running.
373
374 migrate/relocate::
375
376 the service will be relocated (live) to another node.
377
378 remove::
379
380 the service will be removed from the HA managed resource list. Its
381 current state will not be touched.
382
383 start/stop::
384
385 start and stop commands can be issued to the resource specific tools
386 (like 'qm' or 'pct'), they will forward the request to the
387 'ha-manager' which then will execute the action and set the resulting
388 service state (enabled, disabled).
389
390
391 Service States
392 --------------
393
394 stopped::
395
396 Service is stopped (confirmed by LRM)
397
398 request_stop::
399
400 Service should be stopped. Waiting for confirmation from LRM.
401
402 started::
403
404 Service is active an LRM should start it ASAP if not already running.
405
406 fence::
407
408 Wait for node fencing (service node is not inside quorate cluster
409 partition).
410
411 freeze::
412
413 Do not touch the service state. We use this state while we reboot a
414 node, or when we restart the LRM daemon.
415
416 migrate::
417
418 Migrate service (live) to other node.
419
420 error::
421
422 Service disabled because of LRM errors. Needs manual intervention.
423
424
425 ifdef::manvolnum[]
426 include::pve-copyright.adoc[]
427 endif::manvolnum[]
428