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

Ganeti 2.1 design

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

This document describes the major changes in Ganeti 2.1 compared to

the 2.0 version.

The 2.1 version will be a relatively small release. Its main aim is to avoid

changing too much of the core code, while addressing issues and adding new

features and improvements over 2.0, in a timely fashion.

.. contents:: :depth: 4

Objective

=========

Ganeti 2.1 will add features to help further automatization of cluster

operations, further improbe scalability to even bigger clusters, and make it

easier to debug the Ganeti core.

Background

==========

Overview

========

Detailed design

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

As for 2.0 we divide the 2.1 design into three areas:

- core changes, which affect the master daemon/job queue/locking or all/most

  logical units

- logical unit/feature changes

- external interface changes (eg. command line, os api, hooks, ...)

Core changes

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

Storage units modelling

~~~~~~~~~~~~~~~~~~~~~~~

Currently, Ganeti has a good model of the block devices for instances

(e.g. LVM logical volumes, files, DRBD devices, etc.) but none of the

storage pools that are providing the space for these front-end

devices. For example, there are hardcoded inter-node RPC calls for

volume group listing, file storage creation/deletion, etc.

The storage units framework will implement a generic handling for all

kinds of storage backends:

- LVM physical volumes

- LVM volume groups

- File-based storage directories

- any other future storage method

There will be a generic list of methods that each storage unit type

will provide, like:

- list of storage units of this type

- check status of the storage unit

Additionally, there will be specific methods for each method, for example:

- enable/disable allocations on a specific PV

- file storage directory creation/deletion

- VG consistency fixing

This will allow a much better modeling and unification of the various

RPC calls related to backend storage pool in the future. Ganeti 2.1 is

intended to add the basics of the framework, and not necessarilly move

all the curent VG/FileBased operations to it.

Note that while we model both LVM PVs and LVM VGs, the framework will

**not** model any relationship between the different types. In other

words, we don't model neither inheritances nor stacking, since this is

too complex for our needs. While a ``vgreduce`` operation on a LVM VG

could actually remove a PV from it, this will not be handled at the

framework level, but at individual operation level. The goal is that

this is a lightweight framework, for abstracting the different storage

operation, and not for modelling the storage hierarchy.

Locking improvements

~~~~~~~~~~~~~~~~~~~~

Current State and shortcomings

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

The class ``LockSet`` (see ``lib/locking.py``) is a container for one or

many ``SharedLock`` instances. It provides an interface to add/remove locks

and to acquire and subsequently release any number of those locks contained

in it.

Locks in a ``LockSet`` are always acquired in alphabetic order. Due to the

way we're using locks for nodes and instances (the single cluster lock isn't

affected by this issue) this can lead to long delays when acquiring locks if

another operation tries to acquire multiple locks but has to wait for yet

another operation.

In the following demonstration we assume to have the instance locks

``inst1``, ``inst2``, ``inst3`` and ``inst4``.

#. Operation A grabs lock for instance ``inst4``.

#. Operation B wants to acquire all instance locks in alphabetic order, but

   it has to wait for ``inst4``.

#. Operation C tries to lock ``inst1``, but it has to wait until

   Operation B (which is trying to acquire all locks) releases the lock

   again.

#. Operation A finishes and releases lock on ``inst4``. Operation B can

   continue and eventually releases all locks.

#. Operation C can get ``inst1`` lock and finishes.

Technically there's no need for Operation C to wait for Operation A, and

subsequently Operation B, to finish. Operation B can't continue until

Operation A is done (it has to wait for ``inst4``), anyway.

Proposed changes

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

Non-blocking lock acquiring

^^^^^^^^^^^^^^^^^^^^^^^^^^^

Acquiring locks for OpCode execution is always done in blocking mode. They

won't return until the lock has successfully been acquired (or an error

occurred, although we won't cover that case here).

``SharedLock`` and ``LockSet`` must be able to be acquired in a non-blocking

way. They must support a timeout and abort trying to acquire the lock(s)

after the specified amount of time.

Retry acquiring locks

^^^^^^^^^^^^^^^^^^^^^

To prevent other operations from waiting for a long time, such as described

in the demonstration before, ``LockSet`` must not keep locks for a prolonged

period of time when trying to acquire two or more locks. Instead it should,

with an increasing timeout for acquiring all locks, release all locks again

and sleep some time if it fails to acquire all requested locks.

A good timeout value needs to be determined. In any case should ``LockSet``

proceed to acquire locks in blocking mode after a few (unsuccessful)

attempts to acquire all requested locks.

One proposal for the timeout is to use ``2**tries`` seconds, where ``tries``

is the number of unsuccessful tries.

In the demonstration before this would allow Operation C to continue after

Operation B unsuccessfully tried to acquire all locks and released all

acquired locks (``inst1``, ``inst2`` and ``inst3``) again.

Other solutions discussed

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

There was also some discussion on going one step further and extend the job

queue (see ``lib/jqueue.py``) to select the next task for a worker depending

on whether it can acquire the necessary locks. While this may reduce the

number of necessary worker threads and/or increase throughput on large

clusters with many jobs, it also brings many potential problems, such as

contention and increased memory usage, with it. As this would be an

extension of the changes proposed before it could be implemented at a later

point in time, but we decided to stay with the simpler solution for now.

Implementation details

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

``SharedLock`` redesign

^^^^^^^^^^^^^^^^^^^^^^^

The current design of ``SharedLock`` is not good for supporting timeouts

when acquiring a lock and there are also minor fairness issues in it. We

plan to address both with a redesign. A proof of concept implementation was

written and resulted in significantly simpler code.

Currently ``SharedLock`` uses two separate queues for shared and exclusive

acquires and waiters get to run in turns. This means if an exclusive acquire

is released, the lock will allow shared waiters to run and vice versa.

Although it's still fair in the end there is a slight bias towards shared

waiters in the current implementation. The same implementation with two

shared queues can not support timeouts without adding a lot of complexity.

Our proposed redesign changes ``SharedLock`` to have only one single queue.

There will be one condition (see Condition_ for a note about performance) in

the queue per exclusive acquire and two for all shared acquires (see below for

an explanation). The maximum queue length will always be ``2 + (number of

exclusive acquires waiting)``. The number of queue entries for shared acquires

can vary from 0 to 2.

The two conditions for shared acquires are a bit special. They will be used

in turn. When the lock is instantiated, no conditions are in the queue. As

soon as the first shared acquire arrives (and there are holder(s) or waiting

acquires; see Acquire_), the active condition is added to the queue. Until

it becomes the topmost condition in the queue and has been notified, any

shared acquire is added to this active condition. When the active condition

is notified, the conditions are swapped and further shared acquires are

added to the previously inactive condition (which has now become the active

condition). After all waiters on the previously active (now inactive) and

now notified condition received the notification, it is removed from the

queue of pending acquires.

This means shared acquires will skip any exclusive acquire in the queue. We

believe it's better to improve parallelization on operations only asking for

shared (or read-only) locks. Exclusive operations holding the same lock can

not be parallelized.

Acquire

*******

For exclusive acquires a new condition is created and appended to the queue.

Shared acquires are added to the active condition for shared acquires and if

the condition is not yet on the queue, it's appended.

The next step is to wait for our condition to be on the top of the queue (to

guarantee fairness). If the timeout expired, we return to the caller without

acquiring the lock. On every notification we check whether the lock has been

deleted, in which case an error is returned to the caller.

The lock can be acquired if we're on top of the queue (there is no one else

ahead of us). For an exclusive acquire, there must not be other exclusive or

shared holders. For a shared acquire, there must not be an exclusive holder.

If these conditions are all true, the lock is acquired and we return to the

caller. In any other case we wait again on the condition.

If it was the last waiter on a condition, the condition is removed from the

queue.

Optimization: There's no need to touch the queue if there are no pending

acquires and no current holders. The caller can have the lock immediately.

.. image:: design-2.1-lock-acquire.png

Release

*******

First the lock removes the caller from the internal owner list. If there are

pending acquires in the queue, the first (the oldest) condition is notified.

If the first condition was the active condition for shared acquires, the

inactive condition will be made active. This ensures fairness with exclusive

locks by forcing consecutive shared acquires to wait in the queue.

.. image:: design-2.1-lock-release.png

Delete

******

The caller must either hold the lock in exclusive mode already or the lock

must be acquired in exclusive mode. Trying to delete a lock while it's held

in shared mode must fail.

After ensuring the lock is held in exclusive mode, the lock will mark itself

as deleted and continue to notify all pending acquires. They will wake up,

notice the deleted lock and return an error to the caller.

Condition

^^^^^^^^^

Note: This is not necessary for the locking changes above, but it may be a

good optimization (pending performance tests).

The existing locking code in Ganeti 2.0 uses Python's built-in

``threading.Condition`` class. Unfortunately ``Condition`` implements

timeouts by sleeping 1ms to 20ms between tries to acquire the condition lock

in non-blocking mode. This requires unnecessary context switches and

contention on the CPython GIL (Global Interpreter Lock).

By using POSIX pipes (see ``pipe(2)``) we can use the operating system's

support for timeouts on file descriptors (see ``select(2)``). A custom

condition class will have to be written for this.

On instantiation the class creates a pipe. After each notification the

previous pipe is abandoned and re-created (technically the old pipe needs to

stay around until all notifications have been delivered).

All waiting clients of the condition use ``select(2)`` or ``poll(2)`` to

wait for notifications, optionally with a timeout. A notification will be

signalled to the waiting clients by closing the pipe. If the pipe wasn't

closed during the timeout, the waiting function returns to its caller

nonetheless.

Feature changes

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

Ganeti Confd

~~~~~~~~~~~~

Current State and shortcomings

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

In Ganeti 2.0 all nodes are equal, but some are more equal than others. In

particular they are divided between "master", "master candidates" and "normal".

(Moreover they can be offline or drained, but this is not important for the

current discussion). In general the whole configuration is only replicated to

master candidates, and some partial information is spread to all nodes via

ssconf.

This change was done so that the most frequent Ganeti operations didn't need to

contact all nodes, and so clusters could become bigger. If we want more

information to be available on all nodes, we need to add more ssconf values,

which is counter-balancing the change, or to talk with the master node, which

is not designed to happen now, and requires its availability.

Information such as the instance->primary_node mapping will be needed on all

nodes, and we also want to make sure services external to the cluster can query

this information as well. This information must be available at all times, so

we can't query it through RAPI, which would be a single point of failure, as

it's only available on the master.

Proposed changes

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

In order to allow fast and highly available access read-only to some

configuration values, we'll create a new ganeti-confd daemon, which will run on

master candidates. This daemon will talk via UDP, and authenticate messages

using HMAC with a cluster-wide shared key. This key will be generated at

cluster init time, and stored on the clusters alongside the ganeti SSL keys,

and readable only by root.

An interested client can query a value by making a request to a subset of the

cluster master candidates. It will then wait to get a few responses, and use

the one with the highest configuration serial number. Since the configuration

serial number is increased each time the ganeti config is updated, and the

serial number is included in all answers, this can be used to make sure to use

the most recent answer, in case some master candidates are stale or in the

middle of a configuration update.

In order to prevent replay attacks queries will contain the current unix

timestamp according to the client, and the server will verify that its

timestamp is in the same 5 minutes range (this requires synchronized clocks,

which is a good idea anyway). Queries will also contain a "salt" which they

expect the answers to be sent with, and clients are supposed to accept only

answers which contain salt generated by them.

The configuration daemon will be able to answer simple queries such as:

- master candidates list

- master node

- offline nodes

- instance list

- instance primary nodes

Wire protocol

^^^^^^^^^^^^^

A confd query will look like this, on the wire::

  {

    "msg": "{\"type\": 1,

             \"rsalt\": \"9aa6ce92-8336-11de-af38-001d093e835f\",

             \"protocol\": 1,

             \"query\": \"node1.example.com\"}\n",

    "salt": "1249637704",

    "hmac": "4a4139b2c3c5921f7e439469a0a45ad200aead0f"

  }

Detailed explanation of the various fields:

- 'msg' contains a JSON-encoded query, its fields are:

  - 'protocol', integer, is the confd protocol version (initially just

    constants.CONFD_PROTOCOL_VERSION, with a value of 1)

  - 'type', integer, is the query type. For example "node role by name" or

    "node primary ip by instance ip". Constants will be provided for the actual

    available query types.

  - 'query', string, is the search key. For example an ip, or a node name.

  - 'rsalt', string, is the required response salt. The client must use it to

    recognize which answer it's getting.

- 'salt' must be the current unix timestamp, according to the client. Servers

  can refuse messages which have a wrong timing, according to their

  configuration and clock.

- 'hmac' is an hmac signature of salt+msg, with the cluster hmac key

If an answer comes back (which is optional, since confd works over UDP) it will

be in this format::

  {

    "msg": "{\"status\": 0,

             \"answer\": 0,

             \"serial\": 42,

             \"protocol\": 1}\n",

    "salt": "9aa6ce92-8336-11de-af38-001d093e835f",

    "hmac": "aaeccc0dff9328fdf7967cb600b6a80a6a9332af"

  }

Where:

- 'msg' contains a JSON-encoded answer, its fields are:

  - 'protocol', integer, is the confd protocol version (initially just

    constants.CONFD_PROTOCOL_VERSION, with a value of 1)

  - 'status', integer, is the error code. Initially just 0 for 'ok' or '1' for

    'error' (in which case answer contains an error detail, rather than an

    answer), but in the future it may be expanded to have more meanings (eg: 2,

    the answer is compressed)

  - 'answer', is the actual answer. Its type and meaning is query specific. For

    example for "node primary ip by instance ip" queries it will be a string

    containing an IP address, for "node role by name" queries it will be an

    integer which encodes the role (master, candidate, drained, offline)

    according to constants.

- 'salt' is the requested salt from the query. A client can use it to recognize

  what query the answer is answering.

- 'hmac' is an hmac signature of salt+msg, with the cluster hmac key

Redistribute Config

~~~~~~~~~~~~~~~~~~~

Current State and shortcomings

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

Currently LURedistributeConfig triggers a copy of the updated configuration

file to all master candidates and of the ssconf files to all nodes. There are

other files which are maintained manually but which are important to keep in

sync. These are:

- rapi SSL key certificate file (rapi.pem) (on master candidates)

- rapi user/password file rapi_users (on master candidates)

Furthermore there are some files which are hypervisor specific but we may want

to keep in sync:

- the xen-hvm hypervisor uses one shared file for all vnc passwords, and copies

  the file once, during node add. This design is subject to revision to be able

  to have different passwords for different groups of instances via the use of

  hypervisor parameters, and to allow xen-hvm and kvm to use an equal system to

  provide password-protected vnc sessions. In general, though, it would be

  useful if the vnc password files were copied as well, to avoid unwanted vnc

  password changes on instance failover/migrate.

Optionally the admin may want to also ship files such as the global xend.conf

file, and the network scripts to all nodes.

Proposed changes

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

RedistributeConfig will be changed to copy also the rapi files, and to call

every enabled hypervisor asking for a list of additional files to copy. Users

will have the possibility to populate a file containing a list of files to be

distributed; this file will be propagated as well. Such solution is really

simple to implement and it's easily usable by scripts.

This code will be also shared (via tasklets or by other means, if tasklets are

not ready for 2.1) with the AddNode and SetNodeParams LUs (so that the relevant

files will be automatically shipped to new master candidates as they are set).

VNC Console Password

~~~~~~~~~~~~~~~~~~~~

Current State and shortcomings

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

Currently just the xen-hvm hypervisor supports setting a password to connect

the the instances' VNC console, and has one common password stored in a file.

This doesn't allow different passwords for different instances/groups of

instances, and makes it necessary to remember to copy the file around the

cluster when the password changes.

Proposed changes

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

We'll change the VNC password file to a vnc_password_file hypervisor parameter.

This way it can have a cluster default, but also a different value for each

instance. The VNC enabled hypervisors (xen and kvm) will publish all the

password files in use through the cluster so that a redistribute-config will

ship them to all nodes (see the Redistribute Config proposed changes above).

The current VNC_PASSWORD_FILE constant will be removed, but its value will be

used as the default HV_VNC_PASSWORD_FILE value, thus retaining backwards

compatibility with 2.0.

The code to export the list of VNC password files from the hypervisors to

RedistributeConfig will be shared between the KVM and xen-hvm hypervisors.

Disk/Net parameters

~~~~~~~~~~~~~~~~~~~

Current State and shortcomings

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

Currently disks and network interfaces have a few tweakable options and all the

rest is left to a default we chose. We're finding that we need more and more to

tweak some of these parameters, for example to disable barriers for DRBD

devices, or allow striping for the LVM volumes.

Moreover for many of these parameters it will be nice to have cluster-wide

defaults, and then be able to change them per disk/interface.

Proposed changes

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

We will add new cluster level diskparams and netparams, which will contain all

the tweakable parameters. All values which have a sensible cluster-wide default

will go into this new structure while parameters which have unique values will not.

Example of network parameters:

  - mode: bridge/route

  - link: for mode "bridge" the bridge to connect to, for mode route it can

    contain the routing table, or the destination interface

Example of disk parameters:

  - stripe: lvm stripes

  - stripe_size: lvm stripe size

  - meta_flushes: drbd, enable/disable metadata "barriers"

  - data_flushes: drbd, enable/disable data "barriers"

Some parameters are bound to be disk-type specific (drbd, vs lvm, vs files) or

hypervisor specific (nic models for example), but for now they will all live in

the same structure. Each component is supposed to validate only the parameters

it knows about, and ganeti itself will make sure that no "globally unknown"

parameters are added, and that no parameters have overridden meanings for

different components.

The parameters will be kept, as for the BEPARAMS into a "default" category,

which will allow us to expand on by creating instance "classes" in the future.

Instance classes is not a feature we plan implementing in 2.1, though.

Non bridged instances support

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Current State and shortcomings

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

Currently each instance NIC must be connected to a bridge, and if the bridge is

not specified the default cluster one is used. This makes it impossible to use

the vif-route xen network scripts, or other alternative mechanisms that don't

need a bridge to work.

Proposed changes

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

The new "mode" network parameter will distinguish between bridged interfaces

and routed ones.

When mode is "bridge" the "link" parameter will contain the bridge the instance

should be connected to, effectively making things as today. The value has been

migrated from a nic field to a parameter to allow for an easier manipulation of

the cluster default.

When mode is "route" the ip field of the interface will become mandatory, to

allow for a route to be set. In the future we may want also to accept multiple

IPs or IP/mask values for this purpose. We will evaluate possible meanings of

the link parameter to signify a routing table to be used, which would allow for

insulation between instance groups (as today happens for different bridges).

For now we won't add a parameter to specify which network script gets called

for which instance, so in a mixed cluster the network script must be able to

handle both cases. The default kvm vif script will be changed to do so. (Xen

doesn't have a ganeti provided script, so nothing will be done for that

hypervisor)

Introducing persistent UUIDs

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Current state and shortcomings

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

Some objects in the Ganeti configurations are tracked by their name

while also supporting renames. This creates an extra difficulty,

because neither Ganeti nor external management tools can then track

the actual entity, and due to the name change it behaves like a new

one.

Proposed changes part 1

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

We will change Ganeti to use UUIDs for entity tracking, but in a

staggered way. In 2.1, we will simply add an “uuid” attribute to each

of the instances, nodes and cluster itself. This will be reported on

instance creation for nodes, and on node adds for the nodes. It will

be of course avaiblable for querying via the OpQueryNodes/Instance and

cluster information, and via RAPI as well.

Note that Ganeti will not provide any way to change this attribute.

Upgrading from Ganeti 2.0 will automatically add an ‘uuid’ attribute

to all entities missing it.

Proposed changes part 2

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

In the next release (e.g. 2.2), the tracking of objects will change

from the name to the UUID internally, and externally Ganeti will

accept both forms of identification; e.g. an RAPI call would be made

either against ``/2/instances/foo.bar`` or against

``/2/instances/bb3b2e42…``. Since an FQDN must have at least a dot,

and dots are not valid characters in UUIDs, we will not have namespace

issues.

Another change here is that node identification (during cluster

operations/queries like master startup, “am I the master?” and

similar) could be done via UUIDs which is more stable than the current

hostname-based scheme.

Internal tracking refers to the way the configuration is stored; a

DRBD disk of an instance refers to the node name (so that IPs can be

changed easily), but this is still a problem for name changes; thus

these will be changed to point to the node UUID to ease renames.

The advantages of this change (after the second round of changes), is

that node rename becomes trivial, whereas today node rename would

require a complete lock of all instances.

Automated disk repairs infrastructure

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Replacing defective disks in an automated fashion is quite difficult with the

current version of Ganeti. These changes will introduce additional

functionality and interfaces to simplify automating disk replacements on a

Ganeti node.

Fix node volume group

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

This is the most difficult addition, as it can lead to dataloss if it's not

properly safeguarded.

The operation must be done only when all the other nodes that have instances in

common with the target node are fine, i.e. this is the only node with problems,

and also we have to double-check that all instances on this node have at least

a good copy of the data.

This might mean that we have to enhance the GetMirrorStatus calls, and

introduce and a smarter version that can tell us more about the status of an

instance.

Stop allocation on a given PV

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

This is somewhat simple. First we need a "list PVs" opcode (and its associated

logical unit) and then a set PV status opcode/LU. These in combination should

allow both checking and changing the disk/PV status.

Instance disk status

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

This new opcode or opcode change must list the instance-disk-index and node

combinations of the instance together with their status. This will allow

determining what part of the instance is broken (if any).

Repair instance

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

This new opcode/LU/RAPI call will run ``replace-disks -p`` as needed, in order

to fix the instance status. It only affects primary instances; secondaries can

just be moved away.

Migrate node

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

This new opcode/LU/RAPI call will take over the current ``gnt-node migrate``

code and run migrate for all instances on the node.

Evacuate node

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

This new opcode/LU/RAPI call will take over the current ``gnt-node evacuate``

code and run replace-secondary with an iallocator script for all instances on

the node.

External interface changes

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

OS API

~~~~~~

The OS API of Ganeti 2.0 has been built with extensibility in mind. Since we

pass everything as environment variables it's a lot easier to send new

information to the OSes without breaking retrocompatibility. This section of

the design outlines the proposed extensions to the API and their

implementation.

API Version Compatibility Handling

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

In 2.1 there will be a new OS API version (eg. 15), which should be mostly

compatible with api 10, except for some new added variables. Since it's easy

not to pass some variables we'll be able to handle Ganeti 2.0 OSes by just

filtering out the newly added piece of information. We will still encourage

OSes to declare support for the new API after checking that the new variables

don't provide any conflict for them, and we will drop api 10 support after

ganeti 2.1 has released.

New Environment variables

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

Some variables have never been added to the OS api but would definitely be

useful for the OSes. We plan to add an INSTANCE_HYPERVISOR variable to allow

the OS to make changes relevant to the virtualization the instance is going to

use. Since this field is immutable for each instance, the os can tight the

install without caring of making sure the instance can run under any

virtualization technology.

We also want the OS to know the particular hypervisor parameters, to be able to

customize the install even more.  Since the parameters can change, though, we

will pass them only as an "FYI": if an OS ties some instance functionality to

the value of a particular hypervisor parameter manual changes or a reinstall

may be needed to adapt the instance to the new environment. This is not a

regression as of today, because even if the OSes are left blind about this

information, sometimes they still need to make compromises and cannot satisfy

all possible parameter values.

OS Variants

+++++++++++

Currently we are assisting to some degree of "os proliferation" just to change

a simple installation behavior. This means that the same OS gets installed on

the cluster multiple times, with different names, to customize just one

installation behavior. Usually such OSes try to share as much as possible

through symlinks, but this still causes complications on the user side,

especially when multiple parameters must be cross-matched.

For example today if you want to install debian etch, lenny or squeeze you

probably need to install the debootstrap OS multiple times, changing its

configuration file, and calling it debootstrap-etch, debootstrap-lenny or

debootstrap-squeeze. Furthermore if you have for example a "server" and a

"development" environment which installs different packages/configuration files

and must be available for all installs you'll probably end  up with

deboostrap-etch-server, debootstrap-etch-dev, debootrap-lenny-server,

debootstrap-lenny-dev, etc. Crossing more than two parameters quickly becomes

not manageable.

In order to avoid this we plan to make OSes more customizable, by allowing each

OS to declare a list of variants which can be used to customize it. The

variants list is mandatory and must be written, one variant per line, in the

new "variants.list" file inside the main os dir. At least one supported variant

must be supported. When choosing the OS exactly one variant will have to be

specified, and will be encoded in the os name as <OS-name>+<variant>. As for

today it will be possible to change an instance's OS at creation or install

time.

The 2.1 OS list will be the combination of each OS, plus its supported

variants. This will cause the name name proliferation to remain, but at least

the internal OS code will be simplified to just parsing the passed variant,

without the need for symlinks or code duplication.

Also we expect the OSes to declare only "interesting" variants, but to accept

some non-declared ones which a user will be able to pass in by overriding the

checks ganeti does. This will be useful for allowing some variations to be used

without polluting the OS list (per-OS documentation should list all supported

variants). If a variant which is not internally supported is forced through,

the OS scripts should abort.

In the future (post 2.1) we may want to move to full fledged parameters all

orthogonal to each other (for example "architecture" (i386, amd64), "suite"

(lenny, squeeze, ...), etc). (As opposed to the variant, which is a single

parameter, and you need a different variant for all the set of combinations you

want to support).  In this case we envision the variants to be moved inside of

Ganeti and be associated with lists parameter->values associations, which will

then be passed to the OS.

IAllocator changes

~~~~~~~~~~~~~~~~~~

Current State and shortcomings

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

The iallocator interface allows creation of instances without manually

specifying nodes, but instead by specifying plugins which will do the

required computations and produce a valid node list.

However, the interface is quite akward to use:

- one cannot set a 'default' iallocator script

- one cannot use it to easily test if allocation would succeed

- some new functionality, such as rebalancing clusters and calculating

  capacity estimates is needed

Proposed changes

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

There are two area of improvements proposed:

- improving the use of the current interface

- extending the IAllocator API to cover more automation

Default iallocator names

^^^^^^^^^^^^^^^^^^^^^^^^

The cluster will hold, for each type of iallocator, a (possibly empty)

list of modules that will be used automatically.

If the list is empty, the behaviour will remain the same.

If the list has one entry, then ganeti will behave as if

'--iallocator' was specifyed on the command line. I.e. use this

allocator by default. If the user however passed nodes, those will be

used in preference.

If the list has multiple entries, they will be tried in order until

one gives a successful answer.

Dry-run allocation

^^^^^^^^^^^^^^^^^^

The create instance LU will get a new 'dry-run' option that will just

simulate the placement, and return the chosen node-lists after running

all the usual checks.

Cluster balancing

^^^^^^^^^^^^^^^^^

Instance add/removals/moves can create a situation where load on the

nodes is not spread equally. For this, a new iallocator mode will be

implemented called ``balance`` in which the plugin, given the current

cluster state, and a maximum number of operations, will need to

compute the instance relocations needed in order to achieve a "better"

(for whatever the script believes it's better) cluster.

Cluster capacity calculation

^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In this mode, called ``capacity``, given an instance specification and

the current cluster state (similar to the ``allocate`` mode), the

plugin needs to return:

- how many instances can be allocated on the cluster with that specification

- on which nodes these will be allocated (in order)

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