docs: Regions, Zones, Clusters and VM mobility

Add a section in the Admin Guide documenting mobility and failure
domains: Regions, Zones, Clusters.
parent cf9e4c32
......@@ -2536,6 +2536,112 @@ Node10:
All sections: :ref:`Scale out Guide <i-synnefo>`
Regions, Zones and Clusters
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.,
``. Two Regions are most times
located geographically far from each other, e.g. "Europe", "US-East". A Region
comprises multiple Zones.
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.
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
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.
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.
Upgrade Notes
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