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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: 3

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.

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.

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 (which will be always

included in the answer). If some candidates are stale, or we are in the middle

of a configuration update, various master candidates may return different

values, and this should make sure the most recent information is used.

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

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. We also

may want to add a global list of files on the cluster object, which will be

propagated as well, or a hook to calculate them. If we implement this feature

there should be a way to specify whether a file must be shipped to all nodes or

just master candidates.

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)

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 Flavours

+++++++++++

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 flavours which can be used to customize it. The

flavours list is mandatory for new API OSes and must contain at least one

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

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

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

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

without the need for symlinks or code duplication.

Also we expect the OSes to declare only "interesting" flavours, 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

flavours). If a flavour 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 orthogonal

parameters for the OSes. In this case we envision the flavours 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)