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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

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

Feature changes

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

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).

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 Parameters

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

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

arbitrary flags to be passed to them. These will be special "OS parameters"

which will be handled by Ganeti mostly as hypervisor or be parameters. This

slightly complicates the interface, but allows one OS (for example

"debootstrap" to be customizable and not require copies to perform different

cations).

Each OS will be able to declare which parameters it supports by listing them

one per line in a special "parameters" file in the OS dir. The parameters can

have a per-os cluster default, or be specified at instance creation time.  They

will then be passed to the OS scripts as: INSTANCE_OS_PARAMETER_<NAME> with

their specified value. The only value checking that will be performed is that

the os parameter value is a string, with only "normal" characters in it.

It will be impossible to change parameters for an instance, except at reinstall

time. Upon reinstall with a different OS the parameters will be by default

discarded and reset to the default (or passed) values, unless a special

--keep-known-os-parameters flag is passed.