Synnefo » snf-ganeti

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Instance auto-repair

====================

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This is a design document detailing the implementation of self-repair and

recreation of instances in Ganeti. It also discusses ideas that might be useful

for more future self-repair situations.

Current state and shortcomings

==============================

Ganeti currently doesn't do any sort of self-repair or self-recreate of

instances:

- If a drbd instance is broken (its primary of secondary nodes go

  offline or need to be drained) an admin or an external tool must fail

  it over if necessary, and then trigger a disk replacement.

- If a plain instance is broken (or both nodes of a drbd instance are)

  an admin or an external tool must recreate its disk and reinstall it.

Moreover in an oversubscribed cluster operations mentioned above might

fail for lack of capacity until a node is repaired or a new one added.

In this case an external tool would also need to go through any

"pending-recreate" or "pending-repair" instances and fix them.

Proposed changes

================

We'd like to increase the self-repair capabilities of Ganeti, at least

with regards to instances. In order to do so we plan to add mechanisms

to mark an instance as "due for being repaired" and then the relevant

repair to be performed as soon as it's possible, on the cluster.

The self repair will be written as part of ganeti-watcher or as an extra

watcher component that is called less often.

As the first version we'll only handle the case in which an instance

lives on an offline or drained node. In the future we may add more

self-repair capabilities for errors ganeti can detect.

New attributes (or tags)

------------------------

In order to know when to perform a self-repair operation we need to know

whether they are allowed by the cluster administrator.

This can be implemented as either new attributes or tags. Tags could be

acceptable as they would only be read and interpreted by the self-repair tool

(part of the watcher), and not by the ganeti core opcodes and node rpcs. The

following tags would be needed:

ganeti:watcher:autorepair:<type>

++++++++++++++++++++++++++++++++

(instance/nodegroup/cluster)

Allow repairs to happen on an instance that has the tag, or that lives

in a cluster or nodegroup which does. Types of repair are in order of

perceived risk, lower to higher, and each type includes allowing the

operations in the lower ones:

- ``fix-storage`` allows a disk replacement or another operation that

  fixes the instance backend storage without affecting the instance

  itself. This can for example recover from a broken drbd secondary, but

  risks data loss if something is wrong on the primary but the secondary

  was somehow recoverable.

- ``migrate`` allows an instance migration. This can recover from a

  drained primary, but can cause an instance crash in some cases (bugs).

- ``failover`` allows instance reboot on the secondary. This can recover

  from an offline primary, but the instance will lose its running state.

- ``reinstall`` allows disks to be recreated and an instance to be

  reinstalled. This can recover from primary&secondary both being

  offline, or from an offline primary in the case of non-redundant

  instances. It causes data loss.

Each repair type allows all the operations in the previous types, in the

order above, in order to ensure a repair can be completed fully. As such

a repair of a lower type might not be able to proceed if it detects an

error condition that requires a more risky or drastic solution, but

never vice versa (if a worse solution is allowed then so is a better

one).

ganeti:watcher:autorepair:suspend[:<timestamp>]

+++++++++++++++++++++++++++++++++++++++++++++++

(instance/nodegroup/cluster)

If this tag is encountered no autorepair operations will start for the

instance (or for any instance, if present at the cluster or group

level). Any job which already started will be allowed to finish, but

then the autorepair system will not proceed further until this tag is

removed, or the timestamp passes (in which case the tag will be removed

automatically by the watcher).

Note that depending on how this tag is used there might still be race

conditions related to it for an external tool that uses it

programmatically, as no "lock tag" or tag "test-and-set" operation is

present at this time. While this is known we won't solve these race

conditions in the first version.

It might also be useful to easily have an operation that tags all

instances matching a  filter on some charateristic. But again, this

wouldn't be specific to this tag.

ganeti:watcher:repair:pending:<type>:<id>:<timestamp>:<jobs>

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

(instance)

If this tag is present a repair of type ``type`` is pending on the

target instance. This means that either jobs are being run, or it's

waiting for resource availability. ``id`` is the unique id identifying

this repair, ``timestamp`` is the time when this tag was first applied

to this instance for this ``id`` (we will "update" the tag by adding a

"new copy" of it and removing the old version as we run more jobs, but

the timestamp will never change for the same repair)

``jobs`` is the list of jobs already run or being run to repair the

instance. If the instance has just been put in pending state but no job

has run yet, this list is empty.

This tag will be set by ganeti if an equivalent autorepair tag is

present and a a repair is needed, or can be set by an external tool to

request a repair as a "once off".

If multiple instances of this tag are present they will be handled in

order of timestamp.

ganeti:watcher:repair:result:<type>:<id>:<timestamp>:<result>:<jobs>

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

(instance)

If this tag is present a repair of type ``type`` has been performed on

the instance and has been completed by ``timestamp``. The result is

either ``success``, ``failure`` or ``enoperm``, and jobs is a comma

separated list of jobs that were executed for this repair.

An ``enoperm`` result is returned when the repair was brought on until

possible, but the repair type doesn't consent to proceed further.

Possible states, and transitions

--------------------------------

At any point an instance can be in one of the following health states:

Healthy

+++++++

The instance lives on only online nodes. The autorepair system will

never touch these instances. Any ``repair:pending`` tags will be removed

and marked ``success`` with no jobs attached to them.

This state can transition to:

- Needs-repair, repair disallowed (node offlined or drained, no

  autorepair tag)

- Needs-repair, autorepair allowed (node offlined or drained, autorepair

  tag present)

- Suspended (a suspend tag is added)

Suspended

+++++++++

Whenever a ``repair:suspend`` tag is added the autorepair code won't

touch the instance until the timestamp on the tag has passed, if

present. The tag will be removed afterwards (and the instance will

transition to its correct state, depending on its health and other

tags).

Note that when an instance is suspended any pending repair is

interrupted, but jobs which were submitted before the suspension are

allowed to finish.

Needs-repair, repair disallowed

+++++++++++++++++++++++++++++++

The instance lives on an offline or drained node, but no autorepair tag

is set, or the autorepair tag set is of a type not powerful enough to

finish the repair. The autorepair system will never touch these

instances, and they can transition to:

- Healthy (manual repair)

- Pending repair (a ``repair:pending`` tag is added)

- Needs-repair, repair allowed always (an autorepair always tag is added)

- Suspended (a suspend tag is added)

Needs-repair, repair allowed always

+++++++++++++++++++++++++++++++++++

A ``repair:pending`` tag is added, and the instance transitions to the

Pending Repair state. The autorepair tag is preserved.

Of course if a ``repair:suspended`` tag is found no pending tag will be

added, and the instance will instead transition to the Suspended state.

Pending repair

++++++++++++++

When an instance is in this stage the following will happen:

If a ``repair:suspended`` tag is found the instance won't be touched and

moved to the Suspended state. Any jobs which were already running will

be left untouched.

If there are still jobs running related to the instance and scheduled by

this repair they will be given more time to run, and the instance will

be checked again later.  The state transitions to itself.

If no jobs are running and the instance is detected to be healthy, the

``repair:result`` tag will be added, and the current active

``repair:pending`` tag will be removed. It will then transition to the

Healthy state if there are no ``repair:pending`` tags, or to the Pending

state otherwise: there, the instance being healthy, those tags will be

resolved without any operation as well (note that this is the same as

transitioning to the Healthy state, where ``repair:pending`` tags would

also be resolved).

If no jobs are running and the instance still has issues:

- if the last job(s) failed it can either be retried a few times, if

  deemed to be safe, or the repair can transition to the Failed state.

  The ``repair:result`` tag will be added, and the active

  ``repair:pending`` tag will be removed (further ``repair:pending``

  tags will not be able to proceed, as explained by the Failed state,

  until the failure state is cleared)

- if the last job(s) succeeded but there are not enough resources to

  proceed, the state will transition to itself and no jobs are

  scheduled. The tag is left untouched (and later checked again). This

  basically just delays any repairs, the current ``pending`` tag stays

  active, and any others are untouched).

- if the last job(s) succeeded but the repair type cannot allow to

  proceed any further the ``repair:result`` tag is added with an

  ``enoperm`` result, and the current ``repair:pending`` tag is removed.

  The instance is now back to "Needs-repair, repair disallowed",

  "Needs-repair, autorepair allowed", or "Pending" if there is already a

  future tag that can repair the instance.

- if the last job(s) succeeded and the repair can continue new job(s)

  can be submitted, and the ``repair:pending`` tag can be updated.

Failed

++++++

If repairing an instance has failed a ``repair:result:failure`` is

added. The presence of this tag is used to detect that an instance is in

this state, and it will not be touched until the failure is investigated

and the tag is removed.

An external tool or person needs to investigate the state of the

instance and remove this tag when he is sure the instance is repaired

and safe to turn back to the normal autorepair system.

(Alternatively we can use the suspended state (indefinitely or

temporarily) to mark the instance as "not touch" when we think a human

needs to look at it. To be decided).

Repair operation

----------------

Possible repairs are:

- Replace-disks (drbd, if the secondary is down), (or other storage

  specific fixes)

- Migrate (shared storage, rbd, drbd, if the primary is drained)

- Failover (shared storage, rbd, drbd, if the primary is down)

- Recreate disks + reinstall (all nodes down, plain, files or drbd)

Note that more than one of these operations may need to happen before a

full repair is completed (eg. if a drbd primary goes offline first a

failover will happen, then a replce-disks).

The self-repair tool will first take care of all needs-repair instance

that can be brought into ``pending`` state, and transition them as

described above.

Then it will go through any ``repair:pending`` instances and handle them

as described above.

Note that the repair tool MAY "group" instances by performing common

repair jobs for them (eg: node evacuate).

Staging of work

---------------

First version: recreate-disks + reinstall (2.6.1)

Second version: failover and migrate repairs (2.7)

Third version: replace disks repair (2.7 or 2.8)

Future work

===========

One important piece of work will be reporting what the autorepair system

is "thinking" and exporting this in a form that can be read by an

outside user or system. In order to do this we need a better

communication system than embedding this information into tags. This

should be thought in an extensible way that can be used in general for

Ganeti to provide "advisory" information about entities it manages, and

for an external system to "advise" ganeti over what it can do, but in a

less direct manner than submitting individual jobs.

Note that cluster verify checks some errors that are actually instance

specific, (eg. a missing backend disk on a drbd node) or node-specific

(eg. an extra lvm device). If we were to split these into "instance

verify", "node verify" and "cluster verify", then we could easily use

this tool to perform some of those repairs as well.

Finally self-repairs could also be extended to the cluster level, for

example concepts like "N+1 failures", missing master candidates, etc. or

node level for some specific types of errors.

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