Synnefo

Regions, Zones and Clusters

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

Region

------

A Region is a single Synnefo installation, with

Compute/Network/Image/Volume/Object Store services. A Region is associated with

one set of Synnefo DBs (Astakos DB, Pithos DB and Cyclades DB). Every Region has a

distinct set of API endpoints, e.g.,

`https://cloud.example.com/cyclades/compute/v2.0`. Two Regions are most times

located geographically far from each other, e.g. "Europe", "US-East". A Region

comprises multiple Zones.

Zone

----

A Zone is a set of Ganeti clusters, in a potentially geographically distinct

location, e.g. "Athens", "Rome". All clusters have access to the same physical

networks, and are considered a single failure domain, e.g., they access the

network over the same router. A Zone comprises muliple Ganeti clusters.

Ganeti cluster

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

A Ganeti cluster is a set of Ganeti nodes (physical machines). One of the nodes

has the role of "Ganeti master". If this node goes down, another node may

undertake the master role. Ganeti nodes run Virtual Machines (VMs). VMs can live

migrate inside a Ganeti cluster. A Ganeti cluster comprises multiple physical

hardware nodes, most times geographically close to each other.

VM mobility

-----------

VMs may move across Regions, Zones, Ganeti clusters and physical nodes. Before we

describe how that's possible, we will describe the different kinds of moving,

providing the corresponding terminology:

Live migration

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

The act of moving a running VM from physical node to physical node without any

impact on its operation. The VM continues to run on its new physical location,

completely unaffected, and without any service downtime or dropped connections.

Live migration typically requires shared storage and networking between the source

and destination nodes.

Live migration is issued by the administrator in the background and is transparent

to the VM user.

Failover

~~~~~~~~

The act of moving a VM from physical node to physical node by stopping it first on

the source node, then re-starting it on the destination node. There is short

service downtime, during the time the VM boots up, and client connections are

dropped.

Failover is issued by the administrator in the background and the VM user will

experience a reboot.

Snapshot Failover

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

The act of moving a VM from physical node to physical node via a point-in-time

snapshot. That is, stopping a VM on the source node, taking a snapshot, then

creating a new VM from that snapshot.

Snapshot failover is issued by the VM user and not the administrator.

Disaster Recovery

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

In Synnefo terminology, Disaster Recovery is the process of sustaining a disaster

in one datacenter, and ensuring business continuity by performing live migration

or failover of running/existing VMs, or respawning VMs from previously made

snapshots. Based on the method used, this can work inside a single Ganeti cluster,

across Ganeti clusters in the same Zone, or across Zones.

Specifically:

Live migration is only supported inside a single Ganeti cluster. Ganeti supports

live migration between nodes in the same cluster with or without shared storage.

Live migration is done at the Ganeti level and is transparent to Synnefo.

Failover is supported inside a Ganeti cluster, across Ganeti clusters and across

Zones. Ganeti supports failover inside a Ganeti cluster with or without shared

storage, which poses minimum downtime for the VM. Failover inside the same Ganeti

cluster is done at the Ganeti level and is transparent to Synnefo.

Ganeti also provides tools for failing over VMs across different Ganeti clusters,

meaning that one can use them to failover VMs across Ganeti clusters of the same

Zone or across Ganeti clusters of different Zones, thus moving across Zones.

Failing over across different Ganeti clusters requires copying of data, resulting

in longer downtimes, depending on the geographical distance and network between

them. Failover across Ganeti clusters, either in the same or different Zones, is

not transparent to Synnefo and requires manual import of intances at Synnefo level

too, by the administrator.

Snapshot failover supports moving VMs across all domains. It is issued by the VM

user and is done at the Synnefo level without the need of running anything at the

Ganeti level or by the administrator.

In the future Synnefo will also support moving VMs across different Regions.