design-2.0.rst 69.5 KB
Newer Older
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
=================
Ganeti 2.0 design
=================

This document describes the major changes in Ganeti 2.0 compared to
the 1.2 version.

The 2.0 version will constitute a rewrite of the 'core' architecture,
paving the way for additional features in future 2.x versions.

.. contents::

Objective
=========

Ganeti 1.2 has many scalability issues and restrictions due to its
roots as software for managing small and 'static' clusters.

Version 2.0 will attempt to remedy first the scalability issues and
then the restrictions.

Background
==========

Iustin Pop's avatar
Iustin Pop committed
25
While Ganeti 1.2 is usable, it severely limits the flexibility of the
26 27 28 29 30 31 32 33 34 35
cluster administration and imposes a very rigid model. It has the
following main scalability issues:

- only one operation at a time on the cluster [#]_
- poor handling of node failures in the cluster
- mixing hypervisors in a cluster not allowed

It also has a number of artificial restrictions, due to historical design:

- fixed number of disks (two) per instance
Iustin Pop's avatar
Iustin Pop committed
36
- fixed number of NICs
37 38 39 40 41

.. [#] Replace disks will release the lock, but this is an exception
       and not a recommended way to operate

The 2.0 version is intended to address some of these problems, and
Iustin Pop's avatar
Iustin Pop committed
42 43 44 45 46 47 48
create a more flexible code base for future developments.

Among these problems, the single-operation at a time restriction is
biggest issue with the current version of Ganeti. It is such a big
impediment in operating bigger clusters that many times one is tempted
to remove the lock just to do a simple operation like start instance
while an OS installation is running.
49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68

Scalability problems
--------------------

Ganeti 1.2 has a single global lock, which is used for all cluster
operations.  This has been painful at various times, for example:

- It is impossible for two people to efficiently interact with a cluster
  (for example for debugging) at the same time.
- When batch jobs are running it's impossible to do other work (for example
  failovers/fixes) on a cluster.

This poses scalability problems: as clusters grow in node and instance
size it's a lot more likely that operations which one could conceive
should run in parallel (for example because they happen on different
nodes) are actually stalling each other while waiting for the global
lock, without a real reason for that to happen.

One of the main causes of this global lock (beside the higher
difficulty of ensuring data consistency in a more granular lock model)
Iustin Pop's avatar
Iustin Pop committed
69 70 71 72
is the fact that currently there is no long-lived process in Ganeti
that can coordinate multiple operations. Each command tries to acquire
the so called *cmd* lock and when it succeeds, it takes complete
ownership of the cluster configuration and state.
73 74 75 76 77 78 79 80 81 82 83 84 85

Other scalability problems are due the design of the DRBD device
model, which assumed at its creation a low (one to four) number of
instances per node, which is no longer true with today's hardware.

Artificial restrictions
-----------------------

Ganeti 1.2 (and previous versions) have a fixed two-disks, one-NIC per
instance model. This is a purely artificial restrictions, but it
touches multiple areas (configuration, import/export, command line)
that it's more fitted to a major release than a minor one.

Iustin Pop's avatar
Iustin Pop committed
86 87 88 89 90 91 92 93
Architecture issues
-------------------

The fact that each command is a separate process that reads the
cluster state, executes the command, and saves the new state is also
an issue on big clusters where the configuration data for the cluster
begins to be non-trivial in size.

94 95 96 97 98 99 100 101
Overview
========

In order to solve the scalability problems, a rewrite of the core
design of Ganeti is required. While the cluster operations themselves
won't change (e.g. start instance will do the same things, the way
these operations are scheduled internally will change radically.

102 103 104 105 106 107 108 109
The new design will change the cluster architecture to:

.. image:: arch-2.0.png

This differs from the 1.2 architecture by the addition of the master
daemon, which will be the only entity to talk to the node daemons.


110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125
Detailed design
===============

The changes for 2.0 can be split into roughly three areas:

- core changes that affect the design of the software
- features (or restriction removals) but which do not have a wide
  impact on the design
- user-level and API-level changes which translate into differences for
  the operation of the cluster

Core changes
------------

The main changes will be switching from a per-process model to a
daemon based model, where the individual gnt-* commands will be
Iustin Pop's avatar
Iustin Pop committed
126 127 128 129 130 131
clients that talk to this daemon (see `Master daemon`_). This will
allow us to get rid of the global cluster lock for most operations,
having instead a per-object lock (see `Granular locking`_). Also, the
daemon will be able to queue jobs, and this will allow the individual
clients to submit jobs without waiting for them to finish, and also
see the result of old requests (see `Job Queue`_).
132 133 134

Beside these major changes, another 'core' change but that will not be
as visible to the users will be changing the model of object attribute
Iustin Pop's avatar
Iustin Pop committed
135
storage, and separate that into name spaces (such that an Xen PVM
136
instance will not have the Xen HVM parameters). This will allow future
Iustin Pop's avatar
Iustin Pop committed
137 138
flexibility in defining additional parameters. For more details see
`Object parameters`_.
139 140 141

The various changes brought in by the master daemon model and the
read-write RAPI will require changes to the cluster security; we move
Iustin Pop's avatar
Iustin Pop committed
142
away from Twisted and use HTTP(s) for intra- and extra-cluster
143 144 145 146 147 148 149 150 151 152 153 154 155
communications. For more details, see the security document in the
doc/ directory.

Master daemon
~~~~~~~~~~~~~

In Ganeti 2.0, we will have the following *entities*:

- the master daemon (on the master node)
- the node daemon (on all nodes)
- the command line tools (on the master node)
- the RAPI daemon (on the master node)

Iustin Pop's avatar
Iustin Pop committed
156
The master-daemon related interaction paths are:
157

Iustin Pop's avatar
Iustin Pop committed
158
- (CLI tools/RAPI daemon) and the master daemon, via the so called *LUXI* API
159 160
- the master daemon and the node daemons, via the node RPC

Iustin Pop's avatar
Iustin Pop committed
161 162 163 164 165 166 167
There are also some additional interaction paths for exceptional cases:

- CLI tools might access via SSH the nodes (for ``gnt-cluster copyfile``
  and ``gnt-cluster command``)
- master failover is a special case when a non-master node will SSH
  and do node-RPC calls to the current master

168
The protocol between the master daemon and the node daemons will be
Iustin Pop's avatar
Iustin Pop committed
169 170 171 172 173 174 175 176 177 178
changed from (Ganeti 1.2) Twisted PB (perspective broker) to HTTP(S),
using a simple PUT/GET of JSON-encoded messages. This is done due to
difficulties in working with the Twisted framework and its protocols
in a multithreaded environment, which we can overcome by using a
simpler stack (see the caveats section).

The protocol between the CLI/RAPI and the master daemon will be a
custom one (called *LUXI*): on a UNIX socket on the master node, with
rights restricted by filesystem permissions, the CLI/RAPI will talk to
the master daemon using JSON-encoded messages.
179 180 181 182 183 184

The operations supported over this internal protocol will be encoded
via a python library that will expose a simple API for its
users. Internally, the protocol will simply encode all objects in JSON
format and decode them on the receiver side.

Iustin Pop's avatar
Iustin Pop committed
185 186 187
For more details about the RAPI daemon see `Remote API changes`_, and
for the node daemon see `Node daemon changes`_.

188 189 190
The LUXI protocol
+++++++++++++++++

Iustin Pop's avatar
Iustin Pop committed
191 192 193 194 195 196 197 198 199 200
As described above, the protocol for making requests or queries to the
master daemon will be a UNIX-socket based simple RPC of JSON-encoded
messages.

The choice of UNIX was in order to get rid of the need of
authentication and authorisation inside Ganeti; for 2.0, the
permissions on the Unix socket itself will determine the access
rights.

We will have two main classes of operations over this API:
201 202 203 204 205

- cluster query functions
- job related functions

The cluster query functions are usually short-duration, and are the
Iustin Pop's avatar
Iustin Pop committed
206
equivalent of the ``OP_QUERY_*`` opcodes in Ganeti 1.2 (and they are
207 208 209 210 211 212 213 214 215 216
internally implemented still with these opcodes). The clients are
guaranteed to receive the response in a reasonable time via a timeout.

The job-related functions will be:

- submit job
- query job (which could also be categorized in the query-functions)
- archive job (see the job queue design doc)
- wait for job change, which allows a client to wait without polling

Iustin Pop's avatar
Iustin Pop committed
217
For more details of the actual operation list, see the `Job Queue`_.
218

Iustin Pop's avatar
Iustin Pop committed
219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255
Both requests and responses will consist of a JSON-encoded message
followed by the ``ETX`` character (ASCII decimal 3), which is not a
valid character in JSON messages and thus can serve as a message
delimiter. The contents of the messages will be a dictionary with two
fields:

:method:
  the name of the method called
:args:
  the arguments to the method, as a list (no keyword arguments allowed)

Responses will follow the same format, with the two fields being:

:success:
  a boolean denoting the success of the operation
:result:
  the actual result, or error message in case of failure

There are two special value for the result field:

- in the case that the operation failed, and this field is a list of
  length two, the client library will try to interpret is as an exception,
  the first element being the exception type and the second one the
  actual exception arguments; this will allow a simple method of passing
  Ganeti-related exception across the interface
- for the *WaitForChange* call (that waits on the server for a job to
  change status), if the result is equal to ``nochange`` instead of the
  usual result for this call (a list of changes), then the library will
  internally retry the call; this is done in order to differentiate
  internally between master daemon hung and job simply not changed

Users of the API that don't use the provided python library should
take care of the above two cases.


Master daemon implementation
++++++++++++++++++++++++++++
256 257 258 259 260 261 262 263 264 265 266

The daemon will be based around a main I/O thread that will wait for
new requests from the clients, and that does the setup/shutdown of the
other thread (pools).

There will two other classes of threads in the daemon:

- job processing threads, part of a thread pool, and which are
  long-lived, started at daemon startup and terminated only at shutdown
  time
- client I/O threads, which are the ones that talk the local protocol
Iustin Pop's avatar
Iustin Pop committed
267
  (LUXI) to the clients, and are short-lived
268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300

Master startup/failover
+++++++++++++++++++++++

In Ganeti 1.x there is no protection against failing over the master
to a node with stale configuration. In effect, the responsibility of
correct failovers falls on the admin. This is true both for the new
master and for when an old, offline master startup.

Since in 2.x we are extending the cluster state to cover the job queue
and have a daemon that will execute by itself the job queue, we want
to have more resilience for the master role.

The following algorithm will happen whenever a node is ready to
transition to the master role, either at startup time or at node
failover:

#. read the configuration file and parse the node list
   contained within

#. query all the nodes and make sure we obtain an agreement via
   a quorum of at least half plus one nodes for the following:

    - we have the latest configuration and job list (as
      determined by the serial number on the configuration and
      highest job ID on the job queue)

    - there is not even a single node having a newer
      configuration file

    - if we are not failing over (but just starting), the
      quorum agrees that we are the designated master

Iustin Pop's avatar
Iustin Pop committed
301 302 303
    - if any of the above is false, we prevent the current operation
      (i.e. we don't become the master)

304 305 306 307 308
#. at this point, the node transitions to the master role

#. for all the in-progress jobs, mark them as failed, with
   reason unknown or something similar (master failed, etc.)

Iustin Pop's avatar
Iustin Pop committed
309 310 311 312 313 314 315 316
Since due to exceptional conditions we could have a situation in which
no node can become the master due to inconsistent data, we will have
an override switch for the master daemon startup that will assume the
current node has the right data and will replicate all the
configuration files to the other nodes.

**Note**: the above algorithm is by no means an election algorithm; it
is a *confirmation* of the master role currently held by a node.
317 318 319 320

Logging
+++++++

Iustin Pop's avatar
Iustin Pop committed
321 322 323 324
The logging system will be switched completely to the standard python
logging module; currently it's logging-based, but exposes a different
API, which is just overhead. As such, the code will be switched over
to standard logging calls, and only the setup will be custom.
325 326 327 328 329 330 331 332

With this change, we will remove the separate debug/info/error logs,
and instead have always one logfile per daemon model:

- master-daemon.log for the master daemon
- node-daemon.log for the node daemon (this is the same as in 1.2)
- rapi-daemon.log for the RAPI daemon logs
- rapi-access.log, an additional log file for the RAPI that will be
Iustin Pop's avatar
Iustin Pop committed
333 334 335 336 337 338 339 340 341 342 343 344 345
  in the standard HTTP log format for possible parsing by other tools

Since the `watcher`_ will only submit jobs to the master for startup
of the instances, its log file will contain less information than
before, mainly that it will start the instance, but not the results.

Node daemon changes
+++++++++++++++++++

The only change to the node daemon is that, since we need better
concurrency, we don't process the inter-node RPC calls in the node
daemon itself, but we fork and process each request in a separate
child.
346

Iustin Pop's avatar
Iustin Pop committed
347 348
Since we don't have many calls, and we only fork (not exec), the
overhead should be minimal.
349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370

Caveats
+++++++

A discussed alternative is to keep the current individual processes
touching the cluster configuration model. The reasons we have not
chosen this approach is:

- the speed of reading and unserializing the cluster state
  today is not small enough that we can ignore it; the addition of
  the job queue will make the startup cost even higher. While this
  runtime cost is low, it can be on the order of a few seconds on
  bigger clusters, which for very quick commands is comparable to
  the actual duration of the computation itself

- individual commands would make it harder to implement a
  fire-and-forget job request, along the lines "start this
  instance but do not wait for it to finish"; it would require a
  model of backgrounding the operation and other things that are
  much better served by a daemon-based model

Another area of discussion is moving away from Twisted in this new
Iustin Pop's avatar
Iustin Pop committed
371 372
implementation. While Twisted has its advantages, there are also many
disadvantages to using it:
373 374

- first and foremost, it's not a library, but a framework; thus, if
Iustin Pop's avatar
Iustin Pop committed
375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397
  you use twisted, all the code needs to be 'twiste-ized' and written
  in an asynchronous manner, using deferreds; while this method works,
  it's not a common way to code and it requires that the entire process
  workflow is based around a single *reactor* (Twisted name for a main
  loop)
- the more advanced granular locking that we want to implement would
  require, if written in the async-manner, deep integration with the
  Twisted stack, to such an extend that business-logic is inseparable
  from the protocol coding; we felt that this is an unreasonable request,
  and that a good protocol library should allow complete separation of
  low-level protocol calls and business logic; by comparison, the threaded
  approach combined with HTTPs protocol required (for the first iteration)
  absolutely no changes from the 1.2 code, and later changes for optimizing
  the inter-node RPC calls required just syntactic changes (e.g.
  ``rpc.call_...`` to ``self.rpc.call_...``)

Another issue is with the Twisted API stability - during the Ganeti
1.x lifetime, we had to to implement many times workarounds to changes
in the Twisted version, so that for example 1.2 is able to use both
Twisted 2.x and 8.x.

In the end, since we already had an HTTP server library for the RAPI,
we just reused that for inter-node communication.
398 399 400 401 402 403 404 405 406 407 408


Granular locking
~~~~~~~~~~~~~~~~

We want to make sure that multiple operations can run in parallel on a Ganeti
Cluster. In order for this to happen we need to make sure concurrently run
operations don't step on each other toes and break the cluster.

This design addresses how we are going to deal with locking so that:

Iustin Pop's avatar
Iustin Pop committed
409 410 411
- we preserve data coherency
- we prevent deadlocks
- we prevent job starvation
412 413 414 415 416 417

Reaching the maximum possible parallelism is a Non-Goal. We have identified a
set of operations that are currently bottlenecks and need to be parallelised
and have worked on those. In the future it will be possible to address other
needs, thus making the cluster more and more parallel one step at a time.

Iustin Pop's avatar
Iustin Pop committed
418 419
This section only talks about parallelising Ganeti level operations, aka
Logical Units, and the locking needed for that. Any other synchronization lock
420 421
needed internally by the code is outside its scope.

Iustin Pop's avatar
Iustin Pop committed
422 423
Library details
+++++++++++++++
424 425 426

The proposed library has these features:

Iustin Pop's avatar
Iustin Pop committed
427
- internally managing all the locks, making the implementation transparent
428
  from their usage
Iustin Pop's avatar
Iustin Pop committed
429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451
- automatically grabbing multiple locks in the right order (avoid deadlock)
- ability to transparently handle conversion to more granularity
- support asynchronous operation (future goal)

Locking will be valid only on the master node and will not be a
distributed operation. Therefore, in case of master failure, the
operations currently running will be aborted and the locks will be
lost; it remains to the administrator to cleanup (if needed) the
operation result (e.g. make sure an instance is either installed
correctly or removed).

A corollary of this is that a master-failover operation with both
masters alive needs to happen while no operations are running, and
therefore no locks are held.

All the locks will be represented by objects (like
``lockings.SharedLock``), and the individual locks for each object
will be created at initialisation time, from the config file.

The API will have a way to grab one or more than one locks at the same time.
Any attempt to grab a lock while already holding one in the wrong order will be
checked for, and fail.

452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467

The Locks
+++++++++

At the first stage we have decided to provide the following locks:

- One "config file" lock
- One lock per node in the cluster
- One lock per instance in the cluster

All the instance locks will need to be taken before the node locks, and the
node locks before the config lock. Locks will need to be acquired at the same
time for multiple instances and nodes, and internal ordering will be dealt
within the locking library, which, for simplicity, will just use alphabetical
order.

Iustin Pop's avatar
Iustin Pop committed
468 469 470 471 472 473
Each lock has the following three possible statuses:

- unlocked (anyone can grab the lock)
- shared (anyone can grab/have the lock but only in shared mode)
- exclusive (no one else can grab/have the lock)

474 475 476 477 478
Handling conversion to more granularity
+++++++++++++++++++++++++++++++++++++++

In order to convert to a more granular approach transparently each time we
split a lock into more we'll create a "metalock", which will depend on those
Iustin Pop's avatar
Iustin Pop committed
479
sub-locks and live for the time necessary for all the code to convert (or
480 481 482 483 484 485 486
forever, in some conditions). When a metalock exists all converted code must
acquire it in shared mode, so it can run concurrently, but still be exclusive
with old code, which acquires it exclusively.

In the beginning the only such lock will be what replaces the current "command"
lock, and will acquire all the locks in the system, before proceeding. This
lock will be called the "Big Ganeti Lock" because holding that one will avoid
Iustin Pop's avatar
Iustin Pop committed
487
any other concurrent Ganeti operations.
488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508

We might also want to devise more metalocks (eg. all nodes, all nodes+config)
in order to make it easier for some parts of the code to acquire what it needs
without specifying it explicitly.

In the future things like the node locks could become metalocks, should we
decide to split them into an even more fine grained approach, but this will
probably be only after the first 2.0 version has been released.

Adding/Removing locks
+++++++++++++++++++++

When a new instance or a new node is created an associated lock must be added
to the list. The relevant code will need to inform the locking library of such
a change.

This needs to be compatible with every other lock in the system, especially
metalocks that guarantee to grab sets of resources without specifying them
explicitly. The implementation of this will be handled in the locking library
itself.

Iustin Pop's avatar
Iustin Pop committed
509 510 511 512 513
When instances or nodes disappear from the cluster the relevant locks
must be removed. This is easier than adding new elements, as the code
which removes them must own them exclusively already, and thus deals
with metalocks exactly as normal code acquiring those locks. Any
operation queuing on a removed lock will fail after its removal.
514 515 516 517 518 519 520 521 522 523 524

Asynchronous operations
+++++++++++++++++++++++

For the first version the locking library will only export synchronous
operations, which will block till the needed lock are held, and only fail if
the request is impossible or somehow erroneous.

In the future we may want to implement different types of asynchronous
operations such as:

Iustin Pop's avatar
Iustin Pop committed
525 526
- try to acquire this lock set and fail if not possible
- try to acquire one of these lock sets and return the first one you were
527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544
  able to get (or after a timeout) (select/poll like)

These operations can be used to prioritize operations based on available locks,
rather than making them just blindly queue for acquiring them. The inherent
risk, though, is that any code using the first operation, or setting a timeout
for the second one, is susceptible to starvation and thus may never be able to
get the required locks and complete certain tasks. Considering this
providing/using these operations should not be among our first priorities.

Locking granularity
+++++++++++++++++++

For the first version of this code we'll convert each Logical Unit to
acquire/release the locks it needs, so locking will be at the Logical Unit
level.  In the future we may want to split logical units in independent
"tasklets" with their own locking requirements. A different design doc (or mini
design doc) will cover the move from Logical Units to tasklets.

Iustin Pop's avatar
Iustin Pop committed
545 546
Code examples
+++++++++++++
547 548 549 550 551 552 553 554 555 556

In general when acquiring locks we should use a code path equivalent to::

  lock.acquire()
  try:
    ...
    # other code
  finally:
    lock.release()

Iustin Pop's avatar
Iustin Pop committed
557 558 559 560 561
This makes sure we release all locks, and avoid possible deadlocks. Of
course extra care must be used not to leave, if possible locked
structures in an unusable state. Note that with Python 2.5 a simpler
syntax will be possible, but we want to keep compatibility with Python
2.4 so the new constructs should not be used.
562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605

In order to avoid this extra indentation and code changes everywhere in the
Logical Units code, we decided to allow LUs to declare locks, and then execute
their code with their locks acquired. In the new world LUs are called like
this::

  # user passed names are expanded to the internal lock/resource name,
  # then known needed locks are declared
  lu.ExpandNames()
  ... some locking/adding of locks may happen ...
  # late declaration of locks for one level: this is useful because sometimes
  # we can't know which resource we need before locking the previous level
  lu.DeclareLocks() # for each level (cluster, instance, node)
  ... more locking/adding of locks can happen ...
  # these functions are called with the proper locks held
  lu.CheckPrereq()
  lu.Exec()
  ... locks declared for removal are removed, all acquired locks released ...

The Processor and the LogicalUnit class will contain exact documentation on how
locks are supposed to be declared.

Caveats
+++++++

This library will provide an easy upgrade path to bring all the code to
granular locking without breaking everything, and it will also guarantee
against a lot of common errors. Code switching from the old "lock everything"
lock to the new system, though, needs to be carefully scrutinised to be sure it
is really acquiring all the necessary locks, and none has been overlooked or
forgotten.

The code can contain other locks outside of this library, to synchronise other
threaded code (eg for the job queue) but in general these should be leaf locks
or carefully structured non-leaf ones, to avoid deadlock race conditions.


Job Queue
~~~~~~~~~

Granular locking is not enough to speed up operations, we also need a
queue to store these and to be able to process as many as possible in
parallel.

Iustin Pop's avatar
Iustin Pop committed
606
A Ganeti job will consist of multiple ``OpCodes`` which are the basic
607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623
element of operation in Ganeti 1.2 (and will remain as such). Most
command-level commands are equivalent to one OpCode, or in some cases
to a sequence of opcodes, all of the same type (e.g. evacuating a node
will generate N opcodes of type replace disks).


Job execution—“Life of a Ganeti job
++++++++++++++++++++++++++++++++++++

#. Job gets submitted by the client. A new job identifier is generated and
   assigned to the job. The job is then automatically replicated [#replic]_
   to all nodes in the cluster. The identifier is returned to the client.
#. A pool of worker threads waits for new jobs. If all are busy, the job has
   to wait and the first worker finishing its work will grab it. Otherwise any
   of the waiting threads will pick up the new job.
#. Client waits for job status updates by calling a waiting RPC function.
   Log message may be shown to the user. Until the job is started, it can also
Iustin Pop's avatar
Iustin Pop committed
624
   be canceled.
625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758
#. As soon as the job is finished, its final result and status can be retrieved
   from the server.
#. If the client archives the job, it gets moved to a history directory.
   There will be a method to archive all jobs older than a a given age.

.. [#replic] We need replication in order to maintain the consistency across
   all nodes in the system; the master node only differs in the fact that
   now it is running the master daemon, but it if fails and we do a master
   failover, the jobs are still visible on the new master (though marked as
   failed).

Failures to replicate a job to other nodes will be only flagged as
errors in the master daemon log if more than half of the nodes failed,
otherwise we ignore the failure, and rely on the fact that the next
update (for still running jobs) will retry the update. For finished
jobs, it is less of a problem.

Future improvements will look into checking the consistency of the job
list and jobs themselves at master daemon startup.


Job storage
+++++++++++

Jobs are stored in the filesystem as individual files, serialized
using JSON (standard serialization mechanism in Ganeti).

The choice of storing each job in its own file was made because:

- a file can be atomically replaced
- a file can easily be replicated to other nodes
- checking consistency across nodes can be implemented very easily, since
  all job files should be (at a given moment in time) identical

The other possible choices that were discussed and discounted were:

- single big file with all job data: not feasible due to difficult updates
- in-process databases: hard to replicate the entire database to the
  other nodes, and replicating individual operations does not mean wee keep
  consistency


Queue structure
+++++++++++++++

All file operations have to be done atomically by writing to a temporary file
and subsequent renaming. Except for log messages, every change in a job is
stored and replicated to other nodes.

::

  /var/lib/ganeti/queue/
    job-1 (JSON encoded job description and status)
    []
    job-37
    job-38
    job-39
    lock (Queue managing process opens this file in exclusive mode)
    serial (Last job ID used)
    version (Queue format version)


Locking
+++++++

Locking in the job queue is a complicated topic. It is called from more than
one thread and must be thread-safe. For simplicity, a single lock is used for
the whole job queue.

A more detailed description can be found in doc/locking.txt.


Internal RPC
++++++++++++

RPC calls available between Ganeti master and node daemons:

jobqueue_update(file_name, content)
  Writes a file in the job queue directory.
jobqueue_purge()
  Cleans the job queue directory completely, including archived job.
jobqueue_rename(old, new)
  Renames a file in the job queue directory.


Client RPC
++++++++++

RPC between Ganeti clients and the Ganeti master daemon supports the following
operations:

SubmitJob(ops)
  Submits a list of opcodes and returns the job identifier. The identifier is
  guaranteed to be unique during the lifetime of a cluster.
WaitForJobChange(job_id, fields, [], timeout)
  This function waits until a job changes or a timeout expires. The condition
  for when a job changed is defined by the fields passed and the last log
  message received.
QueryJobs(job_ids, fields)
  Returns field values for the job identifiers passed.
CancelJob(job_id)
  Cancels the job specified by identifier. This operation may fail if the job
  is already running, canceled or finished.
ArchiveJob(job_id)
  Moves a job into the /archive/ directory. This operation will fail if the
  job has not been canceled or finished.


Job and opcode status
+++++++++++++++++++++

Each job and each opcode has, at any time, one of the following states:

Queued
  The job/opcode was submitted, but did not yet start.
Waiting
  The job/opcode is waiting for a lock to proceed.
Running
  The job/opcode is running.
Canceled
  The job/opcode was canceled before it started.
Success
  The job/opcode ran and finished successfully.
Error
  The job/opcode was aborted with an error.

If the master is aborted while a job is running, the job will be set to the
Error status once the master started again.


History
+++++++

Archived jobs are kept in a separate directory,
Iustin Pop's avatar
Iustin Pop committed
759 760 761 762
``/var/lib/ganeti/queue/archive/``.  This is done in order to speed up
the queue handling: by default, the jobs in the archive are not
touched by any functions. Only the current (unarchived) jobs are
parsed, loaded, and verified (if implemented) by the master daemon.
763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801


Ganeti updates
++++++++++++++

The queue has to be completely empty for Ganeti updates with changes
in the job queue structure. In order to allow this, there will be a
way to prevent new jobs entering the queue.


Object parameters
~~~~~~~~~~~~~~~~~

Across all cluster configuration data, we have multiple classes of
parameters:

A. cluster-wide parameters (e.g. name of the cluster, the master);
   these are the ones that we have today, and are unchanged from the
   current model

#. node parameters

#. instance specific parameters, e.g. the name of disks (LV), that
   cannot be shared with other instances

#. instance parameters, that are or can be the same for many
   instances, but are not hypervisor related; e.g. the number of VCPUs,
   or the size of memory

#. instance parameters that are hypervisor specific (e.g. kernel_path
   or PAE mode)


The following definitions for instance parameters will be used below:

:hypervisor parameter:
  a hypervisor parameter (or hypervisor specific parameter) is defined
  as a parameter that is interpreted by the hypervisor support code in
  Ganeti and usually is specific to a particular hypervisor (like the
Iustin Pop's avatar
Iustin Pop committed
802
  kernel path for `PVM`_ which makes no sense for `HVM`_).
803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831

:backend parameter:
  a backend parameter is defined as an instance parameter that can be
  shared among a list of instances, and is either generic enough not
  to be tied to a given hypervisor or cannot influence at all the
  hypervisor behaviour.

  For example: memory, vcpus, auto_balance

  All these parameters will be encoded into constants.py with the prefix "BE\_"
  and the whole list of parameters will exist in the set "BES_PARAMETERS"

:proper parameter:
  a parameter whose value is unique to the instance (e.g. the name of a LV,
  or the MAC of a NIC)

As a general rule, for all kind of parameters, None (or in
JSON-speak, nil) will no longer be a valid value for a parameter. As
such, only non-default parameters will be saved as part of objects in
the serialization step, reducing the size of the serialized format.

Cluster parameters
++++++++++++++++++

Cluster parameters remain as today, attributes at the top level of the
Cluster object. In addition, two new attributes at this level will
hold defaults for the instances:

- hvparams, a dictionary indexed by hypervisor type, holding default
Iustin Pop's avatar
Iustin Pop committed
832
  values for hypervisor parameters that are not defined/overridden by
833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858
  the instances of this hypervisor type

- beparams, a dictionary holding (for 2.0) a single element 'default',
  which holds the default value for backend parameters

Node parameters
+++++++++++++++

Node-related parameters are very few, and we will continue using the
same model for these as previously (attributes on the Node object).

Instance parameters
+++++++++++++++++++

As described before, the instance parameters are split in three:
instance proper parameters, unique to each instance, instance
hypervisor parameters and instance backend parameters.

The hvparams and beparams are kept in two dictionaries at instance
level. Only non-default parameters are stored (but once customized, a
parameter will be kept, even with the same value as the default one,
until reset).

The names for hypervisor parameters in the instance.hvparams subtree
should be choosen as generic as possible, especially if specific
parameters could conceivably be useful for more than one hypervisor,
Iustin Pop's avatar
Iustin Pop committed
859 860 861
e.g. ``instance.hvparams.vnc_console_port`` instead of using both
``instance.hvparams.hvm_vnc_console_port`` and
``instance.hvparams.kvm_vnc_console_port``.
862 863

There are some special cases related to disks and NICs (for example):
Iustin Pop's avatar
Iustin Pop committed
864
a disk has both Ganeti-related parameters (e.g. the name of the LV)
865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910
and hypervisor-related parameters (how the disk is presented to/named
in the instance). The former parameters remain as proper-instance
parameters, while the latter value are migrated to the hvparams
structure. In 2.0, we will have only globally-per-instance such
hypervisor parameters, and not per-disk ones (e.g. all NICs will be
exported as of the same type).

Starting from the 1.2 list of instance parameters, here is how they
will be mapped to the three classes of parameters:

- name (P)
- primary_node (P)
- os (P)
- hypervisor (P)
- status (P)
- memory (BE)
- vcpus (BE)
- nics (P)
- disks (P)
- disk_template (P)
- network_port (P)
- kernel_path (HV)
- initrd_path (HV)
- hvm_boot_order (HV)
- hvm_acpi (HV)
- hvm_pae (HV)
- hvm_cdrom_image_path (HV)
- hvm_nic_type (HV)
- hvm_disk_type (HV)
- vnc_bind_address (HV)
- serial_no (P)


Parameter validation
++++++++++++++++++++

To support the new cluster parameter design, additional features will
be required from the hypervisor support implementations in Ganeti.

The hypervisor support  implementation API will be extended with the
following features:

:PARAMETERS: class-level attribute holding the list of valid parameters
  for this hypervisor
:CheckParamSyntax(hvparams): checks that the given parameters are
  valid (as in the names are valid) for this hypervisor; usually just
Iustin Pop's avatar
Iustin Pop committed
911 912 913
  comparing ``hvparams.keys()`` and ``cls.PARAMETERS``; this is a class
  method that can be called from within master code (i.e. cmdlib) and
  should be safe to do so
914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943
:ValidateParameters(hvparams): verifies the values of the provided
  parameters against this hypervisor; this is a method that will be
  called on the target node, from backend.py code, and as such can
  make node-specific checks (e.g. kernel_path checking)

Default value application
+++++++++++++++++++++++++

The application of defaults to an instance is done in the Cluster
object, via two new methods as follows:

- ``Cluster.FillHV(instance)``, returns 'filled' hvparams dict, based on
  instance's hvparams and cluster's ``hvparams[instance.hypervisor]``

- ``Cluster.FillBE(instance, be_type="default")``, which returns the
  beparams dict, based on the instance and cluster beparams

The FillHV/BE transformations will be used, for example, in the RpcRunner
when sending an instance for activation/stop, and the sent instance
hvparams/beparams will have the final value (noded code doesn't know
about defaults).

LU code will need to self-call the transformation, if needed.

Opcode changes
++++++++++++++

The parameter changes will have impact on the OpCodes, especially on
the following ones:

Iustin Pop's avatar
Iustin Pop committed
944
- ``OpCreateInstance``, where the new hv and be parameters will be sent as
945 946
  dictionaries; note that all hv and be parameters are now optional, as
  the values can be instead taken from the cluster
Iustin Pop's avatar
Iustin Pop committed
947
- ``OpQueryInstances``, where we have to be able to query these new
948 949 950 951
  parameters; the syntax for names will be ``hvparam/$NAME`` and
  ``beparam/$NAME`` for querying an individual parameter out of one
  dictionary, and ``hvparams``, respectively ``beparams``, for the whole
  dictionaries
Iustin Pop's avatar
Iustin Pop committed
952
- ``OpModifyInstance``, where the the modified parameters are sent as
953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995
  dictionaries

Additionally, we will need new OpCodes to modify the cluster-level
defaults for the be/hv sets of parameters.

Caveats
+++++++

One problem that might appear is that our classification is not
complete or not good enough, and we'll need to change this model. As
the last resort, we will need to rollback and keep 1.2 style.

Another problem is that classification of one parameter is unclear
(e.g. ``network_port``, is this BE or HV?); in this case we'll take
the risk of having to move parameters later between classes.

Security
++++++++

The only security issue that we foresee is if some new parameters will
have sensitive value. If so, we will need to have a way to export the
config data while purging the sensitive value.

E.g. for the drbd shared secrets, we could export these with the
values replaced by an empty string.

Feature changes
---------------

The main feature-level changes will be:

- a number of disk related changes
- removal of fixed two-disk, one-nic per instance limitation

Disk handling changes
~~~~~~~~~~~~~~~~~~~~~

The storage options available in Ganeti 1.x were introduced based on
then-current software (first DRBD 0.7 then later DRBD 8) and the
estimated usage patters. However, experience has later shown that some
assumptions made initially are not true and that more flexibility is
needed.

Iustin Pop's avatar
Iustin Pop committed
996
One main assumption made was that disk failures should be treated as 'rare'
997 998 999
events, and that each of them needs to be manually handled in order to ensure
data safety; however, both these assumptions are false:

Iustin Pop's avatar
Iustin Pop committed
1000
- disk failures can be a common occurrence, based on usage patterns or cluster
1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060
  size
- our disk setup is robust enough (referring to DRBD8 + LVM) that we could
  automate more of the recovery

Note that we still don't have fully-automated disk recovery as a goal, but our
goal is to reduce the manual work needed.

As such, we plan the following main changes:

- DRBD8 is much more flexible and stable than its previous version (0.7),
  such that removing the support for the ``remote_raid1`` template and
  focusing only on DRBD8 is easier

- dynamic discovery of DRBD devices is not actually needed in a cluster that
  where the DRBD namespace is controlled by Ganeti; switching to a static
  assignment (done at either instance creation time or change secondary time)
  will change the disk activation time from O(n) to O(1), which on big
  clusters is a significant gain

- remove the hard dependency on LVM (currently all available storage types are
  ultimately backed by LVM volumes) by introducing file-based storage

Additionally, a number of smaller enhancements are also planned:
- support variable number of disks
- support read-only disks

Future enhancements in the 2.x series, which do not require base design
changes, might include:

- enhancement of the LVM allocation method in order to try to keep
  all of an instance's virtual disks on the same physical
  disks

- add support for DRBD8 authentication at handshake time in
  order to ensure each device connects to the correct peer

- remove the restrictions on failover only to the secondary
  which creates very strict rules on cluster allocation

DRBD minor allocation
+++++++++++++++++++++

Currently, when trying to identify or activate a new DRBD (or MD)
device, the code scans all in-use devices in order to see if we find
one that looks similar to our parameters and is already in the desired
state or not. Since this needs external commands to be run, it is very
slow when more than a few devices are already present.

Therefore, we will change the discovery model from dynamic to
static. When a new device is logically created (added to the
configuration) a free minor number is computed from the list of
devices that should exist on that node and assigned to that
device.

At device activation, if the minor is already in use, we check if
it has our parameters; if not so, we just destroy the device (if
possible, otherwise we abort) and start it with our own
parameters.

This means that we in effect take ownership of the minor space for
Iustin Pop's avatar
Iustin Pop committed
1061
that device type; if there's a user-created DRBD minor, it will be
1062 1063 1064 1065 1066 1067
automatically removed.

The change will have the effect of reducing the number of external
commands run per device from a constant number times the index of the
first free DRBD minor to just a constant number.

Iustin Pop's avatar
Iustin Pop committed
1068
Removal of obsolete device types (MD, DRBD7)
1069 1070 1071
++++++++++++++++++++++++++++++++++++++++++++

We need to remove these device types because of two issues. First,
Iustin Pop's avatar
Iustin Pop committed
1072
DRBD7 has bad failure modes in case of dual failures (both network and
1073
disk - it cannot propagate the error up the device stack and instead
Iustin Pop's avatar
Iustin Pop committed
1074 1075 1076
just panics. Second, due to the asymmetry between primary and
secondary in MD+DRBD mode, we cannot do live failover (not even if we
had MD+DRBD8).
1077 1078 1079 1080

File-based storage support
++++++++++++++++++++++++++

Iustin Pop's avatar
Iustin Pop committed
1081 1082 1083 1084
Using files instead of logical volumes for instance storage would
allow us to get rid of the hard requirement for volume groups for
testing clusters and it would also allow usage of SAN storage to do
live failover taking advantage of this storage solution.
1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134

Better LVM allocation
+++++++++++++++++++++

Currently, the LV to PV allocation mechanism is a very simple one: at
each new request for a logical volume, tell LVM to allocate the volume
in order based on the amount of free space. This is good for
simplicity and for keeping the usage equally spread over the available
physical disks, however it introduces a problem that an instance could
end up with its (currently) two drives on two physical disks, or
(worse) that the data and metadata for a DRBD device end up on
different drives.

This is bad because it causes unneeded ``replace-disks`` operations in
case of a physical failure.

The solution is to batch allocations for an instance and make the LVM
handling code try to allocate as close as possible all the storage of
one instance. We will still allow the logical volumes to spill over to
additional disks as needed.

Note that this clustered allocation can only be attempted at initial
instance creation, or at change secondary node time. At add disk time,
or at replacing individual disks, it's not easy enough to compute the
current disk map so we'll not attempt the clustering.

DRBD8 peer authentication at handshake
++++++++++++++++++++++++++++++++++++++

DRBD8 has a new feature that allow authentication of the peer at
connect time. We can use this to prevent connecting to the wrong peer
more that securing the connection. Even though we never had issues
with wrong connections, it would be good to implement this.


LVM self-repair (optional)
++++++++++++++++++++++++++

The complete failure of a physical disk is very tedious to
troubleshoot, mainly because of the many failure modes and the many
steps needed. We can safely automate some of the steps, more
specifically the ``vgreduce --removemissing`` using the following
method:

#. check if all nodes have consistent volume groups
#. if yes, and previous status was yes, do nothing
#. if yes, and previous status was no, save status and restart
#. if no, and previous status was no, do nothing
#. if no, and previous status was yes:
    #. if more than one node is inconsistent, do nothing
Iustin Pop's avatar
Iustin Pop committed
1135
    #. if only one node is inconsistent:
1136
        #. run ``vgreduce --removemissing``
Iustin Pop's avatar
Iustin Pop committed
1137
        #. log this occurrence in the Ganeti log in a form that
1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171
           can be used for monitoring
        #. [FUTURE] run ``replace-disks`` for all
           instances affected

Failover to any node
++++++++++++++++++++

With a modified disk activation sequence, we can implement the
*failover to any* functionality, removing many of the layout
restrictions of a cluster:

- the need to reserve memory on the current secondary: this gets reduced to
  a must to reserve memory anywhere on the cluster

- the need to first failover and then replace secondary for an
  instance: with failover-to-any, we can directly failover to
  another node, which also does the replace disks at the same
  step

In the following, we denote the current primary by P1, the current
secondary by S1, and the new primary and secondaries by P2 and S2. P2
is fixed to the node the user chooses, but the choice of S2 can be
made between P1 and S1. This choice can be constrained, depending on
which of P1 and S1 has failed.

- if P1 has failed, then S1 must become S2, and live migration is not possible
- if S1 has failed, then P1 must become S2, and live migration could be
  possible (in theory, but this is not a design goal for 2.0)

The algorithm for performing the failover is straightforward:

- verify that S2 (the node the user has chosen to keep as secondary) has
  valid data (is consistent)

Iustin Pop's avatar
Iustin Pop committed
1172
- tear down the current DRBD association and setup a DRBD pairing between
1173
  P2 (P2 is indicated by the user) and S2; since P2 has no data, it will
Iustin Pop's avatar
Iustin Pop committed
1174
  start re-syncing from S2
1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187

- as soon as P2 is in state SyncTarget (i.e. after the resync has started
  but before it has finished), we can promote it to primary role (r/w)
  and start the instance on P2

- as soon as the P2?S2 sync has finished, we can remove
  the old data on the old node that has not been chosen for
  S2

Caveats: during the P2?S2 sync, a (non-transient) network error
will cause I/O errors on the instance, so (if a longer instance
downtime is acceptable) we can postpone the restart of the instance
until the resync is done. However, disk I/O errors on S2 will cause
Iustin Pop's avatar
Iustin Pop committed
1188
data loss, since we don't have a good copy of the data anymore, so in
1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200
this case waiting for the sync to complete is not an option. As such,
it is recommended that this feature is used only in conjunction with
proper disk monitoring.


Live migration note: While failover-to-any is possible for all choices
of S2, migration-to-any is possible only if we keep P1 as S2.

Caveats
+++++++

The dynamic device model, while more complex, has an advantage: it
Iustin Pop's avatar
Iustin Pop committed
1201 1202
will not reuse by mistake the DRBD device of another instance, since
it always looks for either our own or a free one.
1203 1204 1205 1206 1207

The static one, in contrast, will assume that given a minor number N,
it's ours and we can take over. This needs careful implementation such
that if the minor is in use, either we are able to cleanly shut it
down, or we abort the startup. Otherwise, it could be that we start
Iustin Pop's avatar
Iustin Pop committed
1208
syncing between two instance's disks, causing data loss.
1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219


Variable number of disk/NICs per instance
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Variable number of disks
++++++++++++++++++++++++

In order to support high-security scenarios (for example read-only sda
and read-write sdb), we need to make a fully flexibly disk
definition. This has less impact that it might look at first sight:
Iustin Pop's avatar
Iustin Pop committed
1220
only the instance creation has hard coded number of disks, not the disk
1221 1222 1223 1224 1225 1226 1227
handling code. The block device handling and most of the instance
handling code is already working with "the instance's disks" as
opposed to "the two disks of the instance", but some pieces are not
(e.g. import/export) and the code needs a review to ensure safety.

The objective is to be able to specify the number of disks at
instance creation, and to be able to toggle from read-only to
Iustin Pop's avatar
Iustin Pop committed
1228
read-write a disk afterward.
1229 1230 1231 1232 1233 1234 1235

Variable number of NICs
+++++++++++++++++++++++

Similar to the disk change, we need to allow multiple network
interfaces per instance. This will affect the internal code (some
function will have to stop assuming that ``instance.nics`` is a list
Iustin Pop's avatar
Iustin Pop committed
1236
of length one), the OS API which currently can export/import only one
1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280
instance, and the command line interface.

Interface changes
-----------------

There are two areas of interface changes: API-level changes (the OS
interface and the RAPI interface) and the command line interface
changes.

OS interface
~~~~~~~~~~~~

The current Ganeti OS interface, version 5, is tailored for Ganeti 1.2. The
interface is composed by a series of scripts which get called with certain
parameters to perform OS-dependent operations on the cluster. The current
scripts are:

create
  called when a new instance is added to the cluster
export
  called to export an instance disk to a stream
import
  called to import from a stream to a new instance
rename
  called to perform the os-specific operations necessary for renaming an
  instance

Currently these scripts suffer from the limitations of Ganeti 1.2: for example
they accept exactly one block and one swap devices to operate on, rather than
any amount of generic block devices, they blindly assume that an instance will
have just one network interface to operate, they can not be configured to
optimise the instance for a particular hypervisor.

Since in Ganeti 2.0 we want to support multiple hypervisors, and a non-fixed
number of network and disks the OS interface need to change to transmit the
appropriate amount of information about an instance to its managing operating
system, when operating on it. Moreover since some old assumptions usually used
in OS scripts are no longer valid we need to re-establish a common knowledge on
what can be assumed and what cannot be regarding Ganeti environment.


When designing the new OS API our priorities are:
- ease of use
- future extensibility
Iustin Pop's avatar
Iustin Pop committed
1281
- ease of porting from the old API
1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332
- modularity

As such we want to limit the number of scripts that must be written to support
an OS, and make it easy to share code between them by uniforming their input.
We also will leave the current script structure unchanged, as far as we can,
and make a few of the scripts (import, export and rename) optional. Most
information will be passed to the script through environment variables, for
ease of access and at the same time ease of using only the information a script
needs.


The Scripts
+++++++++++

As in Ganeti 1.2, every OS which wants to be installed in Ganeti needs to
support the following functionality, through scripts:

create:
  used to create a new instance running that OS. This script should prepare the
  block devices, and install them so that the new OS can boot under the
  specified hypervisor.
export (optional):
  used to export an installed instance using the given OS to a format which can
  be used to import it back into a new instance.
import (optional):
  used to import an exported instance into a new one. This script is similar to
  create, but the new instance should have the content of the export, rather
  than contain a pristine installation.
rename (optional):
  used to perform the internal OS-specific operations needed to rename an
  instance.

If any optional script is not implemented Ganeti will refuse to perform the
given operation on instances using the non-implementing OS. Of course the
create script is mandatory, and it doesn't make sense to support the either the
export or the import operation but not both.

Incompatibilities with 1.2
__________________________

We expect the following incompatibilities between the OS scripts for 1.2 and
the ones for 2.0:

- Input parameters: in 1.2 those were passed on the command line, in 2.0 we'll
  use environment variables, as there will be a lot more information and not
  all OSes may care about all of it.
- Number of calls: export scripts will be called once for each device the
  instance has, and import scripts once for every exported disk. Imported
  instances will be forced to have a number of disks greater or equal to the
  one of the export.
- Some scripts are not compulsory: if such a script is missing the relevant
Iustin Pop's avatar
Iustin Pop committed
1333
  operations will be forbidden for instances of that OS. This makes it easier
1334 1335 1336 1337 1338 1339 1340 1341 1342 1343
  to distinguish between unsupported operations and no-op ones (if any).


Input
_____

Rather than using command line flags, as they do now, scripts will accept
inputs from environment variables.  We expect the following input values:

OS_API_VERSION
Iustin Pop's avatar
Iustin Pop committed
1344
  The version of the OS API that the following parameters comply with;
1345 1346 1347 1348 1349 1350
  this is used so that in the future we could have OSes supporting
  multiple versions and thus Ganeti send the proper version in this
  parameter
INSTANCE_NAME
  Name of the instance acted on
HYPERVISOR
Iustin Pop's avatar
Iustin Pop committed
1351
  The hypervisor the instance should run on (e.g. 'xen-pvm', 'xen-hvm', 'kvm')
1352 1353 1354
DISK_COUNT
  The number of disks this instance will have
NIC_COUNT
Iustin Pop's avatar
Iustin Pop committed
1355
  The number of NICs this instance will have
1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372
DISK_<N>_PATH
  Path to the Nth disk.
DISK_<N>_ACCESS
  W if read/write, R if read only. OS scripts are not supposed to touch
  read-only disks, but will be passed them to know.
DISK_<N>_FRONTEND_TYPE
  Type of the disk as seen by the instance. Can be 'scsi', 'ide', 'virtio'
DISK_<N>_BACKEND_TYPE
  Type of the disk as seen from the node. Can be 'block', 'file:loop' or
  'file:blktap'
NIC_<N>_MAC
  Mac address for the Nth network interface
NIC_<N>_IP
  Ip address for the Nth network interface, if available
NIC_<N>_BRIDGE
  Node bridge the Nth network interface will be connected to
NIC_<N>_FRONTEND_TYPE
Iustin Pop's avatar
Iustin Pop committed
1373 1374
  Type of the Nth NIC as seen by the instance. For example 'virtio',
  'rtl8139', etc.
1375 1376 1377 1378
DEBUG_LEVEL
  Whether more out should be produced, for debugging purposes. Currently the
  only valid values are 0 and 1.

Iustin Pop's avatar
Iustin Pop committed
1379 1380 1381 1382
These are only the basic variables we are thinking of now, but more
may come during the implementation and they will be documented in the
``ganeti-os-api`` man page. All these variables will be available to
all scripts.
1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410

Some scripts will need a few more information to work. These will have
per-script variables, such as for example:

OLD_INSTANCE_NAME
  rename: the name the instance should be renamed from.
EXPORT_DEVICE
  export: device to be exported, a snapshot of the actual device. The data must be exported to stdout.
EXPORT_INDEX
  export: sequential number of the instance device targeted.
IMPORT_DEVICE
  import: device to send the data to, part of the new instance. The data must be imported from stdin.
IMPORT_INDEX
  import: sequential number of the instance device targeted.

(Rationale for INSTANCE_NAME as an environment variable: the instance name is
always needed and we could pass it on the command line. On the other hand,
though, this would force scripts to both access the environment and parse the
command line, so we'll move it for uniformity.)


Output/Behaviour
________________

As discussed scripts should only send user-targeted information to stderr. The
create and import scripts are supposed to format/initialise the given block
devices and install the correct instance data. The export script is supposed to
export instance data to stdout in a format understandable by the the import
Iustin Pop's avatar
Iustin Pop committed
1411
script. The data will be compressed by Ganeti, so no compression should be
1412 1413 1414 1415 1416 1417 1418
done. The rename script should only modify the instance's knowledge of what
its name is.

Other declarative style features
++++++++++++++++++++++++++++++++

Similar to Ganeti 1.2, OS specifications will need to provide a
Iustin Pop's avatar
Iustin Pop committed
1419 1420 1421 1422 1423 1424 1425
'ganeti_api_version' containing list of numbers matching the
version(s) of the API they implement. Ganeti itself will always be
compatible with one version of the API and may maintain backwards
compatibility if it's feasible to do so. The numbers are one-per-line,
so an OS supporting both version 5 and version 20 will have a file
containing two lines. This is different from Ganeti 1.2, which only
supported one version number.
1426 1427

In addition to that an OS will be able to declare that it does support only a
Iustin Pop's avatar
Iustin Pop committed
1428
subset of the Ganeti hypervisors, by declaring them in the 'hypervisors' file.
1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448


Caveats/Notes
+++++++++++++

We might want to have a "default" import/export behaviour that just dumps all
disks and restores them. This can save work as most systems will just do this,
while allowing flexibility for different systems.

Environment variables are limited in size, but we expect that there will be
enough space to store the information we need. If we discover that this is not
the case we may want to go to a more complex API such as storing those
information on the filesystem and providing the OS script with the path to a
file where they are encoded in some format.



Remote API changes
~~~~~~~~~~~~~~~~~~

Iustin Pop's avatar
Iustin Pop committed
1449 1450 1451 1452
The first Ganeti remote API (RAPI) was designed and deployed with the
Ganeti 1.2.5 release.  That version provide read-only access to the
cluster state. Fully functional read-write API demands significant
internal changes which will be implemented in version 2.0.
1453

Iustin Pop's avatar
Iustin Pop committed
1454 1455 1456 1457 1458 1459
We decided to go with implementing the Ganeti RAPI in a RESTful way,
which is aligned with key features we looking. It is simple,
stateless, scalable and extensible paradigm of API implementation. As
transport it uses HTTP over SSL, and we are implementing it with JSON
encoding, but in a way it possible to extend and provide any other
one.
1460 1461 1462 1463

Design
++++++

Iustin Pop's avatar
Iustin Pop committed
1464 1465 1466 1467 1468
The Ganeti RAPI is implemented as independent daemon, running on the
same node with the same permission level as Ganeti master
daemon. Communication is done through the LUXI library to the master
daemon. In order to keep communication asynchronous RAPI processes two
types of client requests:
1469

Iustin Pop's avatar
Iustin Pop committed
1470 1471
- queries: server is able to answer immediately
- job submission: some time is required for a useful response
1472

Iustin Pop's avatar
Iustin Pop committed
1473 1474 1475
In the query case requested data send back to client in the HTTP
response body. Typical examples of queries would be: list of nodes,
instances, cluster info, etc.
1476

Iustin Pop's avatar
Iustin Pop committed
1477 1478 1479 1480 1481 1482 1483
In the case of job submission, the client receive a job ID, the
identifier which allows to query the job progress in the job queue
(see `Job Queue`_).

Internally, each exported object has an version identifier, which is
used as a state identifier in the HTTP header E-Tag field for
requests/responses to avoid race conditions.
1484 1485 1486 1487 1488


Resource representation
+++++++++++++++++++++++

Iustin Pop's avatar
Iustin Pop committed
1489 1490 1491
The key difference of using REST instead of others API is that REST
requires separation of services via resources with unique URIs. Each
of them should have limited amount of state and support standard HTTP
1492 1493
methods: GET, POST, DELETE, PUT.

Iustin Pop's avatar
Iustin Pop committed
1494 1495 1496 1497 1498 1499
For example in Ganeti's case we can have a set of URI:

 - ``/{clustername}/instances``
 - ``/{clustername}/instances/{instancename}``
 - ``/{clustername}/instances/{instancename}/tag``
 - ``/{clustername}/tag``
1500

Iustin Pop's avatar
Iustin Pop committed
1501 1502 1503 1504 1505
A GET request to ``/{clustername}/instances`` will return the list of
instances, a POST to ``/{clustername}/instances`` should create a new
instance, a DELETE ``/{clustername}/instances/{instancename}`` should
delete the instance, a GET ``/{clustername}/tag`` should return get
cluster tags.
1506

Iustin Pop's avatar
Iustin Pop committed
1507 1508
Each resource URI will have a version prefix. The resource IDs are to
be determined.
1509

Iustin Pop's avatar
Iustin Pop committed
1510 1511 1512
Internal encoding might be JSON, XML, or any other. The JSON encoding
fits nicely in Ganeti RAPI needs. The client can request a specific
representation via the Accept field in the HTTP header.
1513