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Clarify additional information in the cluster docs

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Joshua Boniface 2020-12-19 03:20:29 -05:00
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@ -41,31 +41,31 @@ PVC is build from a number of other, open source components. The main system its
Virtual machines themselves are run with the Linux KVM subsystem via the Libvirt virtual machine management library. This provides the maximum flexibility and compatibility for running various guest operating systems in multiple modes (fully-virtualized, para-virtualized, virtio-enabled, etc.). Virtual machines themselves are run with the Linux KVM subsystem via the Libvirt virtual machine management library. This provides the maximum flexibility and compatibility for running various guest operating systems in multiple modes (fully-virtualized, para-virtualized, virtio-enabled, etc.).
To manage cluster state, PVC uses Zookeeper. This is an Apache project designed to provide a highly-available and always-consistent key-value database. The various daemons all connect to the distributed Zookeeper database to both obtain details about cluster state, and to manage that state. For instance the node daemon watches Zookeeper for information on what VMs to run, networks to create, etc., while the API writes information to Zookeeper in response to requests. To manage cluster state, PVC uses Zookeeper. This is an Apache project designed to provide a highly-available and always-consistent key-value database. The various daemons all connect to the distributed Zookeeper database to both obtain details about cluster state, and to manage that state. For instance the node daemon watches Zookeeper for information on what VMs to run, networks to create, etc., while the API writes to or reads information from Zookeeper in response to requests. The Zookeeper database is the glue which holds the cluster together.
Additional relational database functionality, specifically for the DNS aggregation subsystem and the VM provisioner, is provided by the PostgreSQL database and the Patroni management tool, which provides automatic clustering and failover for PostgreSQL database instances. Additional relational database functionality, specifically for the managed network DNS aggregation subsystem and the VM provisioner, is provided by the PostgreSQL database system and the Patroni management tool, which provides automatic clustering and failover for PostgreSQL database instances.
Node network routing for managed networks providing EBGP VXLAN and route-learning is provided by FRRouting, a descendant project of Quaaga and GNU Zebra. Node network routing for managed networks providing EBGP VXLAN and route-learning is provided by FRRouting, a descendant project of Quaaga and GNU Zebra. Upstream routers can use this interface to learn routes to cluster networks as well.
The storage subsystem is provided by Ceph, a distributed object-based storage subsystem with extensive scalability, self-managing, and self-healing functionality. The Ceph RBD (Rados Block Device) subsystem is used to provide VM block devices similar to traditional LVM or ZFS zvols, but in a distributed, shared-storage manner. The storage subsystem is provided by Ceph, a distributed object-based storage subsystem with extensive scalability, self-managing, and self-healing functionality. The Ceph RBD (RADOS Block Device) subsystem is used to provide VM block devices similar to traditional LVM or ZFS zvols, but in a distributed, shared-storage manner.
All the components are designed to be run on top of Debian GNU/Linux, specifically Debian 10.X "Buster", with the SystemD system service manager. This OS provides a stable base to run the various other subsystems while remaining truly Free Software, while SystemD provides functionality such as automatic daemon restarting and complex startup/shutdown ordering. All the components are designed to be run on top of Debian GNU/Linux, specifically Debian 10.X "Buster", with the SystemD system service manager. This OS provides a stable base to run the various other subsystems while remaining truly Free Software, while SystemD provides functionality such as automatic daemon restarting and complex startup/shutdown ordering.
## Cluster Architecture ## 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. Ideally, all nodes in a cluster will be identical in specifications, but in some situations mismatched nodes are acceptable, with limitations. 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 and configured to do so; there is no strict 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 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. A subset of the nodes, called "coordinators", are statically configured to provide 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. 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. Nodes can also be upgraded or replaced dynamically and without interrupting the cluster, allowing for seamless hardware maintenance, upgrades, and even replacement, as cluster state configuration is held cluster-wide.
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. 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. Nodes are networked together via a set of statically-configured, simple layer-2 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 "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. It should also contain, or be able to route to, the IPMI BMC management interfaces of the node chassis'.
* 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 "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. * 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. A collapsed cluster+storage configuration may be ideal to simplify management of small clusters, or a split configuration can be used to provide flexbility for large or demanding high-performance clusters - this choice is left to the administrator based on their needs.
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. 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.