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@ -53,13 +53,21 @@ All the components are designed to be run on top of Debian GNU/Linux, specifical
## Cluster Architecture
A PVC cluster is based around "nodes", which are physical servers on which the various daemons, storage, networks, and virtual machines run. Each node is self-contained and is able to perform any and all cluster functions if needed; there is no segmentation of function between different types of physical hosts.
A PVC cluster is based around "nodes", which are physical servers on which the various daemons, storage, networks, and virtual machines run. Each node is self-contained and is able to perform any and all cluster functions if needed; there is no segmentation of function between different types of physical hosts. Ideally, all nodes in a cluster will be identical in specifications, but in some situations mismatched nodes are acceptable, with limitations.
A limited number of nodes, called "coordinators", are statically configured to provide additional services for the cluster. For instance, all databases, FRRouting instances, and Ceph management daemons run only on the set of cluster coordinators. At cluster bootstrap, 1 (testing-only), 3 (small clusters), or 5 (large clusters) nodes may be chosen as the coordinators. Other nodes can then be added as "hypervisor" nodes, which then provide only block device (storage) and VM (compute) functionality by connecting to the set of coordinators. This limits the scaling problem of the databases while ensuring there is still maximum redundancy and resiliency for the core cluster services. Which nodes are designated as coordinators can be changed should the administrator so desire, simply by installing the required software on additional nodes, though this is not recommended (the Ceph system in particular is cumbersome to reconfigure).
A subset of the nodes, called "coordinators", are statically configured to provide additional services for the cluster. For instance, all databases, FRRouting instances, and Ceph management daemons run only on the set of cluster coordinators. At cluster bootstrap, 1 (testing-only), 3 (small clusters), or 5 (large clusters) nodes may be chosen as the coordinators. Other nodes can then be added as "hypervisor" nodes, which then provide only block device (storage) and VM (compute) functionality by connecting to the set of coordinators. This limits the scaling problem of the databases while ensuring there is still maximum redundancy and resiliency for the core cluster services.
Additional nodes can be added to the cluster either as coordinators, or as hypervisors, by adding them to the Ansible configuration and running it against the full set of nodes. Note that the number of coordinators must always be odd, and more than 5 coordinators are normally unnecessary and can cause issues with the database; it is thus normally advisable to add any nodes beyond the initial set as hypervisors instead of coordinators. Nodes can be removed from service, but this is a manual process and should not be attempted unless absolutely required; the Ceph subsystem in particular is sensitive to changes in the coordinator nodes.
During runtime, one coordinator is elected the "primary" for the cluster. This designation can shift dynamically in response to cluster events, or be manually migrated by an administrator. The coordinator takes on a number of roles for which only one host may be active at once, for instance to provide DHCP services to managed client networks or to interface with the API.
Nodes are networked together via a set of statically-configured networks. At a minimum, 2 discrete networks are required, with an optional 3rd. The "upstream" network is the primary network for the nodes, and provides functions such as upstream Internet access, routing to and from the cluster nodes, and management via the API; it may be either a firewalled public or NAT'd RFC1918 network, but should never be exposed directly to the Internet. The "cluster" network is an unrouted RFC1918 network which provides inter-node communication for managed client network traffic (VXLANs), cross-node routing, VM migration and failover, and database replication and access. Finally, though optionally collapsed with the "cluster" network, the "storage" network is another unrouted RFC1918 network which provides a dedicated logical and/or physical link between the nodes for storage traffic, including VM block device storage traffic, inter-OSD replication traffic, and Ceph heartbeat traffic, thus allowing it to be completely isolated from the other networks for maximum performance. With each network is a single "floating" IP address which follows the primary coordinator, providing a single interface to the cluster. Once configured, the cluster is then able to create additional networks of two kinds, "bridged" traditional vLANs and "managed" routed VXLANs, to provide network access to VMs.
Nodes are networked together via a set of statically-configured networks. At a minimum, 2 discrete networks are required, with an optional 3rd.
* The "upstream" network is the primary network for the nodes, and provides functions such as upstream Internet access, routing to and from the cluster nodes, and management via the API; it may be either a firewalled public or NAT'd RFC1918 network, but should never be exposed directly to the Internet.
* The "cluster" network is an unrouted RFC1918 network which provides inter-node communication for managed client network traffic (VXLANs), cross-node routing, VM migration and failover, and database replication and access.
* The "storage" network is another unrouted RFC1918 network which provides a dedicated logical and/or physical link between the nodes for storage traffic, including VM block device storage traffic, inter-OSD replication traffic, and Ceph heartbeat traffic, thus allowing it to be completely isolated from the other networks for maximum performance. This network can be optionally colocated with the "cluster" network, by specifying the same device for both, and can be further combined by specifying the same IP for both to completely collapse the "cluster" and "storage" networks. This may be ideal to simply management of small clusters.
Within each network is a single "floating" IP address which follows the primary coordinator, providing a single interface to the cluster. Once configured, the cluster is then able to create additional networks of two kinds, "bridged" traditional vLANs and "managed" routed VXLANs, to provide network access to VMs.
Further information about the general cluster architecture, including important considerations for node specifications/sizing and network configuration, can be found at the [cluster architecture page](/cluster-architecture).