]>
Commit | Line | Data |
---|---|---|
1 | [[chapter_ha_manager]] | |
2 | ifdef::manvolnum[] | |
3 | ha-manager(1) | |
4 | ============= | |
5 | include::attributes.txt[] | |
6 | :pve-toplevel: | |
7 | ||
8 | NAME | |
9 | ---- | |
10 | ||
11 | ha-manager - Proxmox VE HA Manager | |
12 | ||
13 | SYNOPSIS | |
14 | -------- | |
15 | ||
16 | include::ha-manager.1-synopsis.adoc[] | |
17 | ||
18 | DESCRIPTION | |
19 | ----------- | |
20 | endif::manvolnum[] | |
21 | ifndef::manvolnum[] | |
22 | High Availability | |
23 | ================= | |
24 | include::attributes.txt[] | |
25 | :pve-toplevel: | |
26 | endif::manvolnum[] | |
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 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: | |
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 vendors | |
64 | sell components with higher reliability as ``server'' components - | |
65 | usually at higher price. | |
66 | ||
67 | * Eliminate single point of failure (redundant components) | |
68 | ** use an uninterruptible power supply (UPS) | |
69 | ** use redundant power supplies on the main boards | |
70 | ** use ECC-RAM | |
71 | ** use redundant network hardware | |
72 | ** use RAID for local storage | |
73 | ** use distributed, redundant storage for VM data | |
74 | ||
75 | * Reduce downtime | |
76 | ** rapidly accessible administrators (24/7) | |
77 | ** availability of spare parts (other nodes in a {pve} cluster) | |
78 | ** automatic error detection (provided by `ha-manager`) | |
79 | ** automatic failover (provided by `ha-manager`) | |
80 | ||
81 | Virtualization environments like {pve} make it much easier to reach | |
82 | high availability because they remove the ``hardware'' dependency. They | |
83 | also support to setup and use redundant storage and network | |
84 | devices. So if one host fail, you can simply start those services on | |
85 | another host within your cluster. | |
86 | ||
87 | Even better, {pve} provides a software stack called `ha-manager`, | |
88 | which can do that automatically for you. It is able to automatically | |
89 | detect errors and do automatic failover. | |
90 | ||
91 | {pve} `ha-manager` works like an ``automated'' administrator. First, you | |
92 | configure what resources (VMs, containers, ...) it should | |
93 | manage. `ha-manager` then observes correct functionality, and handles | |
94 | service failover to another node in case of errors. `ha-manager` can | |
95 | also handle normal user requests which may start, stop, relocate and | |
96 | migrate a service. | |
97 | ||
98 | But high availability comes at a price. High quality components are | |
99 | more expensive, and making them redundant duplicates the costs at | |
100 | least. Additional spare parts increase costs further. So you should | |
101 | carefully calculate the benefits, and compare with those additional | |
102 | costs. | |
103 | ||
104 | TIP: Increasing availability from 99% to 99.9% is relatively | |
105 | simply. But increasing availability from 99.9999% to 99.99999% is very | |
106 | hard and costly. `ha-manager` has typical error detection and failover | |
107 | times of about 2 minutes, so you can get no more than 99.999% | |
108 | availability. | |
109 | ||
110 | Requirements | |
111 | ------------ | |
112 | ||
113 | * at least three cluster nodes (to get reliable quorum) | |
114 | ||
115 | * shared storage for VMs and containers | |
116 | ||
117 | * hardware redundancy (everywhere) | |
118 | ||
119 | * hardware watchdog - if not available we fall back to the | |
120 | linux kernel software watchdog (`softdog`) | |
121 | ||
122 | * optional hardware fencing devices | |
123 | ||
124 | ||
125 | [[ha_manager_resources]] | |
126 | Resources | |
127 | --------- | |
128 | ||
129 | We call the primary management unit handled by `ha-manager` a | |
130 | resource. A resource (also called ``service'') is uniquely | |
131 | identified by a service ID (SID), which consists of the resource type | |
132 | and an type specific ID, e.g.: `vm:100`. That example would be a | |
133 | resource of type `vm` (virtual machine) with the ID 100. | |
134 | ||
135 | For now we have two important resources types - virtual machines and | |
136 | containers. One basic idea here is that we can bundle related software | |
137 | into such VM or container, so there is no need to compose one big | |
138 | service from other services, like it was done with `rgmanager`. In | |
139 | general, a HA enabled resource should not depend on other resources. | |
140 | ||
141 | ||
142 | How It Works | |
143 | ------------ | |
144 | ||
145 | This section provides an in detail description of the {PVE} HA-manager | |
146 | internals. It describes how the CRM and the LRM work together. | |
147 | ||
148 | To provide High Availability two daemons run on each node: | |
149 | ||
150 | `pve-ha-lrm`:: | |
151 | ||
152 | The local resource manager (LRM), it controls the services running on | |
153 | the local node. | |
154 | It reads the requested states for its services from the current manager | |
155 | status file and executes the respective commands. | |
156 | ||
157 | `pve-ha-crm`:: | |
158 | ||
159 | The cluster resource manager (CRM), it controls the cluster wide | |
160 | actions of the services, processes the LRM results and includes the state | |
161 | machine which controls the state of each service. | |
162 | ||
163 | .Locks in the LRM & CRM | |
164 | [NOTE] | |
165 | Locks are provided by our distributed configuration file system (pmxcfs). | |
166 | They are used to guarantee that each LRM is active once and working. As a | |
167 | LRM only executes actions when it holds its lock we can mark a failed node | |
168 | as fenced if we can acquire its lock. This lets us then recover any failed | |
169 | HA services securely without any interference from the now unknown failed node. | |
170 | This all gets supervised by the CRM which holds currently the manager master | |
171 | lock. | |
172 | ||
173 | Local Resource Manager | |
174 | ~~~~~~~~~~~~~~~~~~~~~~ | |
175 | ||
176 | The local resource manager (`pve-ha-lrm`) is started as a daemon on | |
177 | boot and waits until the HA cluster is quorate and thus cluster wide | |
178 | locks are working. | |
179 | ||
180 | It can be in three states: | |
181 | ||
182 | wait for agent lock:: | |
183 | ||
184 | The LRM waits for our exclusive lock. This is also used as idle state if no | |
185 | service is configured. | |
186 | ||
187 | active:: | |
188 | ||
189 | The LRM holds its exclusive lock and has services configured. | |
190 | ||
191 | lost agent lock:: | |
192 | ||
193 | The LRM lost its lock, this means a failure happened and quorum was lost. | |
194 | ||
195 | After the LRM gets in the active state it reads the manager status | |
196 | file in `/etc/pve/ha/manager_status` and determines the commands it | |
197 | has to execute for the services it owns. | |
198 | For each command a worker gets started, this workers are running in | |
199 | parallel and are limited to at most 4 by default. This default setting | |
200 | may be changed through the datacenter configuration key `max_worker`. | |
201 | When finished the worker process gets collected and its result saved for | |
202 | the CRM. | |
203 | ||
204 | .Maximum Concurrent Worker Adjustment Tips | |
205 | [NOTE] | |
206 | The default value of at most 4 concurrent workers may be unsuited for | |
207 | a specific setup. For example may 4 live migrations happen at the same | |
208 | time, which can lead to network congestions with slower networks and/or | |
209 | big (memory wise) services. Ensure that also in the worst case no congestion | |
210 | happens and lower the `max_worker` value if needed. In the contrary, if you | |
211 | have a particularly powerful high end setup you may also want to increase it. | |
212 | ||
213 | Each command requested by the CRM is uniquely identifiable by an UID, when | |
214 | the worker finished its result will be processed and written in the LRM | |
215 | status file `/etc/pve/nodes/<nodename>/lrm_status`. There the CRM may collect | |
216 | it and let its state machine - respective the commands output - act on it. | |
217 | ||
218 | The actions on each service between CRM and LRM are normally always synced. | |
219 | This means that the CRM requests a state uniquely marked by an UID, the LRM | |
220 | then executes this action *one time* and writes back the result, also | |
221 | identifiable by the same UID. This is needed so that the LRM does not | |
222 | executes an outdated command. | |
223 | With the exception of the `stop` and the `error` command, | |
224 | those two do not depend on the result produced and are executed | |
225 | always in the case of the stopped state and once in the case of | |
226 | the error state. | |
227 | ||
228 | .Read the Logs | |
229 | [NOTE] | |
230 | The HA Stack logs every action it makes. This helps to understand what | |
231 | and also why something happens in the cluster. Here its important to see | |
232 | what both daemons, the LRM and the CRM, did. You may use | |
233 | `journalctl -u pve-ha-lrm` on the node(s) where the service is and | |
234 | the same command for the pve-ha-crm on the node which is the current master. | |
235 | ||
236 | Cluster Resource Manager | |
237 | ~~~~~~~~~~~~~~~~~~~~~~~~ | |
238 | ||
239 | The cluster resource manager (`pve-ha-crm`) starts on each node and | |
240 | waits there for the manager lock, which can only be held by one node | |
241 | at a time. The node which successfully acquires the manager lock gets | |
242 | promoted to the CRM master. | |
243 | ||
244 | It can be in three states: | |
245 | ||
246 | wait for agent lock:: | |
247 | ||
248 | The CRM waits for our exclusive lock. This is also used as idle state if no | |
249 | service is configured | |
250 | ||
251 | active:: | |
252 | ||
253 | The CRM holds its exclusive lock and has services configured | |
254 | ||
255 | lost agent lock:: | |
256 | ||
257 | The CRM lost its lock, this means a failure happened and quorum was lost. | |
258 | ||
259 | It main task is to manage the services which are configured to be highly | |
260 | available and try to always enforce them to the wanted state, e.g.: a | |
261 | enabled service will be started if its not running, if it crashes it will | |
262 | be started again. Thus it dictates the LRM the actions it needs to execute. | |
263 | ||
264 | When an node leaves the cluster quorum, its state changes to unknown. | |
265 | If the current CRM then can secure the failed nodes lock, the services | |
266 | will be 'stolen' and restarted on another node. | |
267 | ||
268 | When a cluster member determines that it is no longer in the cluster | |
269 | quorum, the LRM waits for a new quorum to form. As long as there is no | |
270 | quorum the node cannot reset the watchdog. This will trigger a reboot | |
271 | after the watchdog then times out, this happens after 60 seconds. | |
272 | ||
273 | Configuration | |
274 | ------------- | |
275 | ||
276 | The HA stack is well integrated in the Proxmox VE API2. So, for | |
277 | example, HA can be configured via `ha-manager` or the PVE web | |
278 | interface, which both provide an easy to use tool. | |
279 | ||
280 | The resource configuration file can be located at | |
281 | `/etc/pve/ha/resources.cfg` and the group configuration file at | |
282 | `/etc/pve/ha/groups.cfg`. Use the provided tools to make changes, | |
283 | there shouldn't be any need to edit them manually. | |
284 | ||
285 | Node Power Status | |
286 | ----------------- | |
287 | ||
288 | If a node needs maintenance you should migrate and or relocate all | |
289 | services which are required to run always on another node first. | |
290 | After that you can stop the LRM and CRM services. But note that the | |
291 | watchdog triggers if you stop it with active services. | |
292 | ||
293 | Package Updates | |
294 | --------------- | |
295 | ||
296 | When updating the ha-manager you should do one node after the other, never | |
297 | all at once for various reasons. First, while we test our software | |
298 | thoughtfully, a bug affecting your specific setup cannot totally be ruled out. | |
299 | Upgrading one node after the other and checking the functionality of each node | |
300 | after finishing the update helps to recover from an eventual problems, while | |
301 | updating all could render you in a broken cluster state and is generally not | |
302 | good practice. | |
303 | ||
304 | Also, the {pve} HA stack uses a request acknowledge protocol to perform | |
305 | actions between the cluster and the local resource manager. For restarting, | |
306 | the LRM makes a request to the CRM to freeze all its services. This prevents | |
307 | that they get touched by the Cluster during the short time the LRM is restarting. | |
308 | After that the LRM may safely close the watchdog during a restart. | |
309 | Such a restart happens on a update and as already stated a active master | |
310 | CRM is needed to acknowledge the requests from the LRM, if this is not the case | |
311 | the update process can be too long which, in the worst case, may result in | |
312 | a watchdog reset. | |
313 | ||
314 | ||
315 | [[ha_manager_fencing]] | |
316 | Fencing | |
317 | ------- | |
318 | ||
319 | What is Fencing | |
320 | ~~~~~~~~~~~~~~~ | |
321 | ||
322 | Fencing secures that on a node failure the dangerous node gets will be rendered | |
323 | unable to do any damage and that no resource runs twice when it gets recovered | |
324 | from the failed node. This is a really important task and one of the base | |
325 | principles to make a system Highly Available. | |
326 | ||
327 | If a node would not get fenced it would be in an unknown state where it may | |
328 | have still access to shared resources, this is really dangerous! | |
329 | Imagine that every network but the storage one broke, now while not | |
330 | reachable from the public network the VM still runs and writes on the shared | |
331 | storage. If we would not fence the node and just start up this VM on another | |
332 | Node we would get dangerous race conditions, atomicity violations the whole VM | |
333 | could be rendered unusable. The recovery could also simply fail if the storage | |
334 | protects from multiple mounts and thus defeat the purpose of HA. | |
335 | ||
336 | How {pve} Fences | |
337 | ~~~~~~~~~~~~~~~~~ | |
338 | ||
339 | There are different methods to fence a node, for example fence devices which | |
340 | cut off the power from the node or disable their communication completely. | |
341 | ||
342 | Those are often quite expensive and bring additional critical components in | |
343 | a system, because if they fail you cannot recover any service. | |
344 | ||
345 | We thus wanted to integrate a simpler method in the HA Manager first, namely | |
346 | self fencing with watchdogs. | |
347 | ||
348 | Watchdogs are widely used in critical and dependable systems since the | |
349 | beginning of micro controllers, they are often independent and simple | |
350 | integrated circuit which programs can use to watch them. After opening they need to | |
351 | report periodically. If, for whatever reason, a program becomes unable to do | |
352 | so the watchdogs triggers a reset of the whole server. | |
353 | ||
354 | Server motherboards often already include such hardware watchdogs, these need | |
355 | to be configured. If no watchdog is available or configured we fall back to the | |
356 | Linux Kernel softdog while still reliable it is not independent of the servers | |
357 | Hardware and thus has a lower reliability then a hardware watchdog. | |
358 | ||
359 | Configure Hardware Watchdog | |
360 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
361 | By default all watchdog modules are blocked for security reasons as they are | |
362 | like a loaded gun if not correctly initialized. | |
363 | If you have a hardware watchdog available remove its kernel module from the | |
364 | blacklist, load it with insmod and restart the `watchdog-mux` service or reboot | |
365 | the node. | |
366 | ||
367 | Recover Fenced Services | |
368 | ~~~~~~~~~~~~~~~~~~~~~~~ | |
369 | ||
370 | After a node failed and its fencing was successful we start to recover services | |
371 | to other available nodes and restart them there so that they can provide service | |
372 | again. | |
373 | ||
374 | The selection of the node on which the services gets recovered is influenced | |
375 | by the users group settings, the currently active nodes and their respective | |
376 | active service count. | |
377 | First we build a set out of the intersection between user selected nodes and | |
378 | available nodes. Then the subset with the highest priority of those nodes | |
379 | gets chosen as possible nodes for recovery. We select the node with the | |
380 | currently lowest active service count as a new node for the service. | |
381 | That minimizes the possibility of an overload, which else could cause an | |
382 | unresponsive node and as a result a chain reaction of node failures in the | |
383 | cluster. | |
384 | ||
385 | [[ha_manager_groups]] | |
386 | Groups | |
387 | ------ | |
388 | ||
389 | A group is a collection of cluster nodes which a service may be bound to. | |
390 | ||
391 | Group Settings | |
392 | ~~~~~~~~~~~~~~ | |
393 | ||
394 | nodes:: | |
395 | ||
396 | List of group node members where a priority can be given to each node. | |
397 | A service bound to this group will run on the nodes with the highest priority | |
398 | available. If more nodes are in the highest priority class the services will | |
399 | get distributed to those node if not already there. The priorities have a | |
400 | relative meaning only. | |
401 | Example;; | |
402 | You want to run all services from a group on `node1` if possible. If this node | |
403 | is not available, you want them to run equally splitted on `node2` and `node3`, and | |
404 | if those fail it should use `node4`. | |
405 | To achieve this you could set the node list to: | |
406 | [source,bash] | |
407 | ha-manager groupset mygroup -nodes "node1:2,node2:1,node3:1,node4" | |
408 | ||
409 | restricted:: | |
410 | ||
411 | Resources bound to this group may only run on nodes defined by the | |
412 | group. If no group node member is available the resource will be | |
413 | placed in the stopped state. | |
414 | Example;; | |
415 | Lets say a service uses resources only available on `node1` and `node2`, | |
416 | so we need to make sure that HA manager does not use other nodes. | |
417 | We need to create a 'restricted' group with said nodes: | |
418 | [source,bash] | |
419 | ha-manager groupset mygroup -nodes "node1,node2" -restricted | |
420 | ||
421 | nofailback:: | |
422 | ||
423 | The resource won't automatically fail back when a more preferred node | |
424 | (re)joins the cluster. | |
425 | Examples;; | |
426 | * You need to migrate a service to a node which hasn't the highest priority | |
427 | in the group at the moment, to tell the HA manager to not move this service | |
428 | instantly back set the 'nofailback' option and the service will stay on | |
429 | the current node. | |
430 | ||
431 | * A service was fenced and it got recovered to another node. The admin | |
432 | repaired the node and brought it up online again but does not want that the | |
433 | recovered services move straight back to the repaired node as he wants to | |
434 | first investigate the failure cause and check if it runs stable. He can use | |
435 | the 'nofailback' option to achieve this. | |
436 | ||
437 | ||
438 | Start Failure Policy | |
439 | --------------------- | |
440 | ||
441 | The start failure policy comes in effect if a service failed to start on a | |
442 | node once ore more times. It can be used to configure how often a restart | |
443 | should be triggered on the same node and how often a service should be | |
444 | relocated so that it gets a try to be started on another node. | |
445 | The aim of this policy is to circumvent temporary unavailability of shared | |
446 | resources on a specific node. For example, if a shared storage isn't available | |
447 | on a quorate node anymore, e.g. network problems, but still on other nodes, | |
448 | the relocate policy allows then that the service gets started nonetheless. | |
449 | ||
450 | There are two service start recover policy settings which can be configured | |
451 | specific for each resource. | |
452 | ||
453 | max_restart:: | |
454 | ||
455 | Maximum number of tries to restart an failed service on the actual | |
456 | node. The default is set to one. | |
457 | ||
458 | max_relocate:: | |
459 | ||
460 | Maximum number of tries to relocate the service to a different node. | |
461 | A relocate only happens after the max_restart value is exceeded on the | |
462 | actual node. The default is set to one. | |
463 | ||
464 | NOTE: The relocate count state will only reset to zero when the | |
465 | service had at least one successful start. That means if a service is | |
466 | re-enabled without fixing the error only the restart policy gets | |
467 | repeated. | |
468 | ||
469 | Error Recovery | |
470 | -------------- | |
471 | ||
472 | If after all tries the service state could not be recovered it gets | |
473 | placed in an error state. In this state the service won't get touched | |
474 | by the HA stack anymore. To recover from this state you should follow | |
475 | these steps: | |
476 | ||
477 | * bring the resource back into a safe and consistent state (e.g., | |
478 | killing its process) | |
479 | ||
480 | * disable the ha resource to place it in an stopped state | |
481 | ||
482 | * fix the error which led to this failures | |
483 | ||
484 | * *after* you fixed all errors you may enable the service again | |
485 | ||
486 | ||
487 | Service Operations | |
488 | ------------------ | |
489 | ||
490 | This are how the basic user-initiated service operations (via | |
491 | `ha-manager`) work. | |
492 | ||
493 | enable:: | |
494 | ||
495 | The service will be started by the LRM if not already running. | |
496 | ||
497 | disable:: | |
498 | ||
499 | The service will be stopped by the LRM if running. | |
500 | ||
501 | migrate/relocate:: | |
502 | ||
503 | The service will be relocated (live) to another node. | |
504 | ||
505 | remove:: | |
506 | ||
507 | The service will be removed from the HA managed resource list. Its | |
508 | current state will not be touched. | |
509 | ||
510 | start/stop:: | |
511 | ||
512 | `start` and `stop` commands can be issued to the resource specific tools | |
513 | (like `qm` or `pct`), they will forward the request to the | |
514 | `ha-manager` which then will execute the action and set the resulting | |
515 | service state (enabled, disabled). | |
516 | ||
517 | ||
518 | Service States | |
519 | -------------- | |
520 | ||
521 | stopped:: | |
522 | ||
523 | Service is stopped (confirmed by LRM), if detected running it will get stopped | |
524 | again. | |
525 | ||
526 | request_stop:: | |
527 | ||
528 | Service should be stopped. Waiting for confirmation from LRM. | |
529 | ||
530 | started:: | |
531 | ||
532 | Service is active an LRM should start it ASAP if not already running. | |
533 | If the Service fails and is detected to be not running the LRM restarts it. | |
534 | ||
535 | fence:: | |
536 | ||
537 | Wait for node fencing (service node is not inside quorate cluster | |
538 | partition). | |
539 | As soon as node gets fenced successfully the service will be recovered to | |
540 | another node, if possible. | |
541 | ||
542 | freeze:: | |
543 | ||
544 | Do not touch the service state. We use this state while we reboot a | |
545 | node, or when we restart the LRM daemon. | |
546 | ||
547 | migrate:: | |
548 | ||
549 | Migrate service (live) to other node. | |
550 | ||
551 | error:: | |
552 | ||
553 | Service disabled because of LRM errors. Needs manual intervention. | |
554 | ||
555 | ||
556 | ifdef::manvolnum[] | |
557 | include::pve-copyright.adoc[] | |
558 | endif::manvolnum[] | |
559 |