5 .. contents:: :depth: 4
7 This is a design document detailing the implementation of self-repair and
8 recreation of instances in Ganeti. It also discusses ideas that might be useful
9 for more future self-repair situations.
11 Current state and shortcomings
12 ==============================
14 Ganeti currently doesn't do any sort of self-repair or self-recreate of
17 - If a drbd instance is broken (its primary of secondary nodes go
18 offline or need to be drained) an admin or an external tool must fail
19 it over if necessary, and then trigger a disk replacement.
20 - If a plain instance is broken (or both nodes of a drbd instance are)
21 an admin or an external tool must recreate its disk and reinstall it.
23 Moreover in an oversubscribed cluster operations mentioned above might
24 fail for lack of capacity until a node is repaired or a new one added.
25 In this case an external tool would also need to go through any
26 "pending-recreate" or "pending-repair" instances and fix them.
31 We'd like to increase the self-repair capabilities of Ganeti, at least
32 with regards to instances. In order to do so we plan to add mechanisms
33 to mark an instance as "due for being repaired" and then the relevant
34 repair to be performed as soon as it's possible, on the cluster.
36 The self repair will be written as part of ganeti-watcher or as an extra
37 watcher component that is called less often.
39 As the first version we'll only handle the case in which an instance
40 lives on an offline or drained node. In the future we may add more
41 self-repair capabilities for errors ganeti can detect.
43 New attributes (or tags)
44 ------------------------
46 In order to know when to perform a self-repair operation we need to know
47 whether they are allowed by the cluster administrator.
49 This can be implemented as either new attributes or tags. Tags could be
50 acceptable as they would only be read and interpreted by the self-repair tool
51 (part of the watcher), and not by the ganeti core opcodes and node rpcs. The
52 following tags would be needed:
54 ganeti:watcher:autorepair:<type>
55 ++++++++++++++++++++++++++++++++
57 (instance/nodegroup/cluster)
58 Allow repairs to happen on an instance that has the tag, or that lives
59 in a cluster or nodegroup which does. Types of repair are in order of
60 perceived risk, lower to higher, and each type includes allowing the
61 operations in the lower ones:
63 - ``fix-storage`` allows a disk replacement or another operation that
64 fixes the instance backend storage without affecting the instance
65 itself. This can for example recover from a broken drbd secondary, but
66 risks data loss if something is wrong on the primary but the secondary
67 was somehow recoverable.
68 - ``migrate`` allows an instance migration. This can recover from a
69 drained primary, but can cause an instance crash in some cases (bugs).
70 - ``failover`` allows instance reboot on the secondary. This can recover
71 from an offline primary, but the instance will lose its running state.
72 - ``reinstall`` allows disks to be recreated and an instance to be
73 reinstalled. This can recover from primary&secondary both being
74 offline, or from an offline primary in the case of non-redundant
75 instances. It causes data loss.
77 Each repair type allows all the operations in the previous types, in the
78 order above, in order to ensure a repair can be completed fully. As such
79 a repair of a lower type might not be able to proceed if it detects an
80 error condition that requires a more risky or drastic solution, but
81 never vice versa (if a worse solution is allowed then so is a better
84 ganeti:watcher:autorepair:suspend[:<timestamp>]
85 +++++++++++++++++++++++++++++++++++++++++++++++
87 (instance/nodegroup/cluster)
88 If this tag is encountered no autorepair operations will start for the
89 instance (or for any instance, if present at the cluster or group
90 level). Any job which already started will be allowed to finish, but
91 then the autorepair system will not proceed further until this tag is
92 removed, or the timestamp passes (in which case the tag will be removed
93 automatically by the watcher).
95 Note that depending on how this tag is used there might still be race
96 conditions related to it for an external tool that uses it
97 programmatically, as no "lock tag" or tag "test-and-set" operation is
98 present at this time. While this is known we won't solve these race
99 conditions in the first version.
101 It might also be useful to easily have an operation that tags all
102 instances matching a filter on some charateristic. But again, this
103 wouldn't be specific to this tag.
105 ganeti:watcher:repair:pending:<type>:<id>:<timestamp>:<jobs>
106 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
109 If this tag is present a repair of type ``type`` is pending on the
110 target instance. This means that either jobs are being run, or it's
111 waiting for resource availability. ``id`` is the unique id identifying
112 this repair, ``timestamp`` is the time when this tag was first applied
113 to this instance for this ``id`` (we will "update" the tag by adding a
114 "new copy" of it and removing the old version as we run more jobs, but
115 the timestamp will never change for the same repair)
117 ``jobs`` is the list of jobs already run or being run to repair the
118 instance. If the instance has just been put in pending state but no job
119 has run yet, this list is empty.
121 This tag will be set by ganeti if an equivalent autorepair tag is
122 present and a a repair is needed, or can be set by an external tool to
123 request a repair as a "once off".
125 If multiple instances of this tag are present they will be handled in
128 ganeti:watcher:repair:result:<type>:<id>:<timestamp>:<result>:<jobs>
129 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
132 If this tag is present a repair of type ``type`` has been performed on
133 the instance and has been completed by ``timestamp``. The result is
134 either ``success``, ``failure`` or ``enoperm``, and jobs is a comma
135 separated list of jobs that were executed for this repair.
137 An ``enoperm`` result is returned when the repair was brought on until
138 possible, but the repair type doesn't consent to proceed further.
140 Possible states, and transitions
141 --------------------------------
143 At any point an instance can be in one of the following health states:
148 The instance lives on only online nodes. The autorepair system will
149 never touch these instances. Any ``repair:pending`` tags will be removed
150 and marked ``success`` with no jobs attached to them.
152 This state can transition to:
154 - Needs-repair, repair disallowed (node offlined or drained, no
156 - Needs-repair, autorepair allowed (node offlined or drained, autorepair
158 - Suspended (a suspend tag is added)
163 Whenever a ``repair:suspend`` tag is added the autorepair code won't
164 touch the instance until the timestamp on the tag has passed, if
165 present. The tag will be removed afterwards (and the instance will
166 transition to its correct state, depending on its health and other
169 Note that when an instance is suspended any pending repair is
170 interrupted, but jobs which were submitted before the suspension are
173 Needs-repair, repair disallowed
174 +++++++++++++++++++++++++++++++
176 The instance lives on an offline or drained node, but no autorepair tag
177 is set, or the autorepair tag set is of a type not powerful enough to
178 finish the repair. The autorepair system will never touch these
179 instances, and they can transition to:
181 - Healthy (manual repair)
182 - Pending repair (a ``repair:pending`` tag is added)
183 - Needs-repair, repair allowed always (an autorepair always tag is added)
184 - Suspended (a suspend tag is added)
186 Needs-repair, repair allowed always
187 +++++++++++++++++++++++++++++++++++
189 A ``repair:pending`` tag is added, and the instance transitions to the
190 Pending Repair state. The autorepair tag is preserved.
192 Of course if a ``repair:suspended`` tag is found no pending tag will be
193 added, and the instance will instead transition to the Suspended state.
198 When an instance is in this stage the following will happen:
200 If a ``repair:suspended`` tag is found the instance won't be touched and
201 moved to the Suspended state. Any jobs which were already running will
204 If there are still jobs running related to the instance and scheduled by
205 this repair they will be given more time to run, and the instance will
206 be checked again later. The state transitions to itself.
208 If no jobs are running and the instance is detected to be healthy, the
209 ``repair:result`` tag will be added, and the current active
210 ``repair:pending`` tag will be removed. It will then transition to the
211 Healthy state if there are no ``repair:pending`` tags, or to the Pending
212 state otherwise: there, the instance being healthy, those tags will be
213 resolved without any operation as well (note that this is the same as
214 transitioning to the Healthy state, where ``repair:pending`` tags would
217 If no jobs are running and the instance still has issues:
219 - if the last job(s) failed it can either be retried a few times, if
220 deemed to be safe, or the repair can transition to the Failed state.
221 The ``repair:result`` tag will be added, and the active
222 ``repair:pending`` tag will be removed (further ``repair:pending``
223 tags will not be able to proceed, as explained by the Failed state,
224 until the failure state is cleared)
225 - if the last job(s) succeeded but there are not enough resources to
226 proceed, the state will transition to itself and no jobs are
227 scheduled. The tag is left untouched (and later checked again). This
228 basically just delays any repairs, the current ``pending`` tag stays
229 active, and any others are untouched).
230 - if the last job(s) succeeded but the repair type cannot allow to
231 proceed any further the ``repair:result`` tag is added with an
232 ``enoperm`` result, and the current ``repair:pending`` tag is removed.
233 The instance is now back to "Needs-repair, repair disallowed",
234 "Needs-repair, autorepair allowed", or "Pending" if there is already a
235 future tag that can repair the instance.
236 - if the last job(s) succeeded and the repair can continue new job(s)
237 can be submitted, and the ``repair:pending`` tag can be updated.
242 If repairing an instance has failed a ``repair:result:failure`` is
243 added. The presence of this tag is used to detect that an instance is in
244 this state, and it will not be touched until the failure is investigated
245 and the tag is removed.
247 An external tool or person needs to investigate the state of the
248 instance and remove this tag when he is sure the instance is repaired
249 and safe to turn back to the normal autorepair system.
251 (Alternatively we can use the suspended state (indefinitely or
252 temporarily) to mark the instance as "not touch" when we think a human
253 needs to look at it. To be decided).
258 Possible repairs are:
260 - Replace-disks (drbd, if the secondary is down), (or other storage
262 - Migrate (shared storage, rbd, drbd, if the primary is drained)
263 - Failover (shared storage, rbd, drbd, if the primary is down)
264 - Recreate disks + reinstall (all nodes down, plain, files or drbd)
266 Note that more than one of these operations may need to happen before a
267 full repair is completed (eg. if a drbd primary goes offline first a
268 failover will happen, then a replce-disks).
270 The self-repair tool will first take care of all needs-repair instance
271 that can be brought into ``pending`` state, and transition them as
274 Then it will go through any ``repair:pending`` instances and handle them
277 Note that the repair tool MAY "group" instances by performing common
278 repair jobs for them (eg: node evacuate).
283 First version: recreate-disks + reinstall (2.6.1)
284 Second version: failover and migrate repairs (2.7)
285 Third version: replace disks repair (2.7 or 2.8)
290 One important piece of work will be reporting what the autorepair system
291 is "thinking" and exporting this in a form that can be read by an
292 outside user or system. In order to do this we need a better
293 communication system than embedding this information into tags. This
294 should be thought in an extensible way that can be used in general for
295 Ganeti to provide "advisory" information about entities it manages, and
296 for an external system to "advise" ganeti over what it can do, but in a
297 less direct manner than submitting individual jobs.
299 Note that cluster verify checks some errors that are actually instance
300 specific, (eg. a missing backend disk on a drbd node) or node-specific
301 (eg. an extra lvm device). If we were to split these into "instance
302 verify", "node verify" and "cluster verify", then we could easily use
303 this tool to perform some of those repairs as well.
305 Finally self-repairs could also be extended to the cluster level, for
306 example concepts like "N+1 failures", missing master candidates, etc. or
307 node level for some specific types of errors.
309 .. vim: set textwidth=72 :
projects / ganeti-local / blob