pvc/docs/architecture/cluster.md

PVC Cluster Architecture considerations

• PVC Cluster Architecture considerations
  • Node Specifications: Considering the size of nodes
  • Storage Layout: Ceph and OSDs
  • Physical network considerations
  • Network Layout: Considering the required networks
    • PVC system networks
      • Upstream: Connecting the nodes to the wider world
      • Cluster: Connecting the nodes with each other
      • Storage: Connecting Ceph OSD with each other
    • PVC client networks
      • Bridged (unmanaged) Client Networks
      • VXLAN (managed) Client Networks
      • Other Client Networks
  • Node Layout: Considering how nodes are laid out
    • Node Functions: Coordinators versus Hypervisors
      • Coordinators
        • The Primary Coordinator
      • Hypervisors
    • Geographic redundancy
  • Example Configurations
    • Basic 3-node cluster
    • Mid-sized 8-node cluster with 3 coordinators
    • Large 17-node cluster with 5 coordinators

This document contains considerations the administrator should make when preparing for and building a PVC cluster. It is important that prospective PVC administrators read this document thoroughly before deploying a cluster to ensure they understand the requirements, caveats, and important details about how PVC operates.

Node Specifications: Considering the size of nodes

PVC nodes, especially coordinator nodes, run a significant number of software applications in addition to the virtual machines (VMs). It is therefore extremely important to size the systems correctly for the expected workload while planning both for redundancy and future capacity. In general, taller nodes are better for performance, providing a more powerful cluster on fewer physical machines, though each workload may be different in this regard.

The following table provides bare-minimum, recommended, and optimal specifications for a cluster. The bare-minimum specification would be suitable for testing or a small lab, but not for production use. The recommended specification would be suitable for a small production cluster running lightweight VMs. The optimal cluster would be the ideal for running a demanding, resource-intensive production cluster. Note that these are the minimum resources required, and actual usage will likely require more resources than those presented here - this is mostly to show the minimums for each specified configuration (i.e. testing, light production, heavy production).

Resource	Minimum	Recommended	Optimal
CPU generation	Intel Nehalem (2008) / AMD Bulldozer (2011)	Intel Sandy Bridge (2011) / AMD Naples (2017)	Intel Haswell (2013) / AMD Rome (2019)
CPU cores (per node)	4x @1.8GHz	8x @2.0GHz	12x @2.2 GHz
RAM (per node)	16GB	48GB	64GB
System disk (SSD/HDD/USB/SD/eMMC)	1x 10GB	2x 10GB RAID-1	2x 32GB RAID-1
Data disk (SSD only)	1x 200GB	1x 400GB	2x 400GB
Network interfaces	1x 1Gbps	2x 1Gbps LAG	2x 10Gbps LAG
Total CPU cores (healthy)	12x	24x	36x
Total CPU cores (n-1)	8x	16x	24x
Total RAM (healthy)	48GB	144GB	192GB
Total RAM (n-1)	32GB	96GB	128GB
Total disk space	200GB	400GB	800GB

Of these totals, some amount of CPU and RAM will be used by the storage subsystem and the PVC daemons themselves, meaning that the total available for virtual machines is slightly less. Generally, each OSD data disk will consume 1 vCPU at load and 1-2GB RAM, so nodes should be sized not only according to the VM workload, but the number of storage disks per node. Additionally the coordinator databases will use additional RAM and CPU resources of up to 1-4GB per node, though there is generally little need to spec coordinators any larger than non-coordinator nodes and the VM automatic node selection process will take used RAM into account by default.

Care should also be taken to examine the "healthy" versus "n-1" total resource availability. Under normal operation, PVC will use all available resources and distribute VMs across all cluster nodes. However, during single-node failure or maintenance conditions, all VMs will be required to run on the remaining two hypervisors. Thus, care should be taken not to exceed the "n-1" quantity, plus approximately 15-20%, to prevent overloading other nodes in these cases. When in doubt, it is always safe to treat the "n-1" numbers as the total maximum cluster resource availability and plan accordingly. The general rule can be though of as "1/3 of the total disks space, 2/3 of the total RAM, 2/3 of the total CPUs".

As an underlying OS, only Debian 10 "Buster" is supported by PVC. This is the operating system installed by the PVC node installer and expected by the PVC Ansible configuration system. Ubuntu or other Debian-derived distributions may work, but are not officially supported.

Currently, only the amd64 (Intel 64 or AMD64) architecture is officially supported by PVC. Given the cross-platform nature of Python and the various software components in Debian, it may work on armhf or arm64 systems such as Raspbian as well, however this has not been tested by the author.

Storage Layout: Ceph and OSDs

The Ceph subsystem of PVC, if enabled, creates a "hyperconverged" cluster whereby storage and VM hypervisor functions are collocated onto the same physical servers. The performance of the storage must be taken into account when sizing the nodes as mentioned above.

The Ceph system is laid out similar to the other daemons. The Ceph Monitor and Manager functions are delegated to the Coordinators over the storage network, with all nodes connecting to these hosts to obtain the CRUSH maps and select OSD disks. OSDs are then distributed on all hosts, including non-coordinator hypervisors, and communicate with clients and each other over the storage network.

PVC Ceph pools make use of the replication mechanism of Ceph to store multiple copies of each object, thus ensuring that data is always available even when a host is unavailable. Only "replica"-based Ceph redundancy is supported by PVC; erasure coded pools are not supported due to major performance impacts related to rewrites and random I/O.

The default replication level for a new pool is copies=3, mincopies=2. This will store 3 copies of each object, with a host-level failure domain, and will allow I/O as long as 2 copies are available. Thus, in a cluster of any size, all data is fully available even if a single host becomes unavailable. It will however use 3x the space for each piece of data stored, which must be considered when sizing the disk space for the cluster: a pool in this configuration, running on 3 nodes each with a single 400GB disk, will effectively have 400GB of total space available for use. Additionally, new disks must be added in groups of 3 spread across the nodes in order to be able to take advantage of the additional space, since each write will require creating 3 copies across each of the 3 hosts.

Non-default values can also be set at pool creation time. For instance, one could create a copies=3, mincopies=1 pool, which would allow I/O with two hosts down but leaves the cluster susceptible to a write hole should a disk fail in this state. Alternatively, for more resilience, one could create a copies=4, mincopies=2 pool, which will allow 2 hosts to fail without a write hole, but would consume 4x the space for each piece of data stored and require new disks to be added in groups of 4 instead. Practically any combination of values is possible, however these 3 are the most relevant for most use-cases, and for most, especially small, clusters, the default is sufficient to provide solid redundancy and guard against host failures until the administrator can respond.

Replication levels cannot be changed within PVC once a pool is created, however they can be changed via manual Ceph commands on a coordinator should the administrator require this. In any case, the administrator should carefully consider sizing, failure domains, and performance when selecting storage devices to ensure the right level of resiliency versus data usage for their use-case and cluster size.

Physical network considerations

At a minimum, a production PVC cluster should use at least two 1Gbps Ethernet interfaces, connected in an LACP or active-backup bond on one or more switches. On top of this bond, the various cluster networks are be configured as 802.3q vLANs. PVC is be able to support configurations without 802.1q vLAN support using multiple physical interfaces and no bridged client networks, but this is strongly discouraged; the switches chosen for the cluster should include these requirements as a minimum.

More advanced physical network layouts are also possible. For instance, one could have two isolated networks. On the first network, each node has two 10Gbps Ethernet interfaces, which are combined in a bond across two redundant switch fabrics and that handle the upstream and cluster networks. On the second network, each node has an additional two 10Gbps, which are also combined in a bond across the redundant switch fabrics and handle the storage network. This configuration could support up to 10Gbps of aggregate client traffic while also supporting 10Gbps of aggregate storage traffic. Even more complex network configurations are possible if the cluster requires such performance. See the Example Configurations section for some examples.

Network Layout: Considering the required networks

A PVC cluster needs several different networks to operate properly; they are described in detail below and the administrator should ensure they account for all the required networks when planning the cluster.

PVC system networks

Upstream: Connecting the nodes to the wider world

The upstream network functions as the main upstream for the cluster nodes, providing Internet access and a way to route managed client network traffic out of the cluster. In most deployments, this should be an RFC1918 private subnet with an upstream router which can perform NAT translation and firewalling as required, both for the cluster nodes themselves, but also for the RFC1918 managed client networks.

The floating IP address in the cluster network can be used as a single point of communication with the active primary node, for instance to access the DNS aggregator instance or the API if configured. For this reason the network should generally be protected from unauthorized access via a firewall.

Nodes in this network are generally assigned static IP addresses which are configured at node install time and in the Ansible deployment configuration.

The upstream router should be able to handle static routes to the PVC cluster, or form a BGP neighbour relationship with the coordinator nodes and/or floating IP address to learn routes to the managed client networks.

The upstream network should generally be large enough to contain:

0. The upstream router(s)
1. The nodes themselves
2. In most deployments, the node IPMI management interfaces.

For example, for a 3+ node cluster, up to about 90 nodes, the following configuration might be used:

Description	Address
Upstream network	10.0.0.0/24
Router VIP address	10.0.0.1
Router 1 address	10.0.0.2
Router 2 address	10.0.0.3
PVC floating address	10.0.0.10
node1	10.0.0.11
node2	10.0.0.12
etc.	etc.
node1-ipmi	10.0.0.111
node2-ipmi	10.0.0.112
etc.	etc.

For even larger clusters, a /23 or even larger network may be used.

Cluster: Connecting the nodes with each other

The cluster network is an unrouted private network used by the PVC nodes to communicate with each other for database access and Libvirt migrations. It is also used as the underlying interface for the BGP EVPN VXLAN interfaces used by managed client networks.

The floating IP address in the cluster network can be used as a single point of communication with the active primary node.

Nodes in this network are generally assigned IPs automatically based on their node number (e.g. node1 at .1, node2 at .2, etc.). The network should be large enough to include all nodes sequentially.

Generally the cluster network should be completely separate from the upstream network, either a separate physical interface (or set of bonded interfaces) or a dedicated vLAN on an underlying physical device, but they can be collocated if required.

Storage: Connecting Ceph OSD with each other

The storage network is an unrouted private network used by the PVC node storage OSDs to communicated with each other, for Ceph management functionality, and for QEMU-to-Ceph disk access, without using the main cluster network and introducing potentially large amounts of traffic there.

The floating IP address in the storage network can be used as a single point of communication with the active primary node, though this will generally be of little use.

Nodes in this network are generally assigned IPs automatically based on their node number (e.g. node1 at .1, node2 at .2, etc.). The network should be large enough to include all nodes sequentially.

The administrator may choose to collocate the storage network on the same physical interface as the cluster network, or on a separate physical interface. This should be decided based on the size of the cluster and the perceived ratios of client network versus storage traffic. In large (>3 node) or storage-intensive clusters, this network should generally be a separate set of fast physical interfaces, separate from both the upstream and cluster networks, in order to maximize and isolate the storage bandwidth. If the administrator does choose to colocate these networks, they may also share the same IP address, thus eliminating any distinction between the Cluster and Storage networks. The PVC software handles this natively when the Cluster and Storage IPs of a node are identical.

PVC client networks

Bridged (unmanaged) Client Networks

The first type of client network is the unmanaged bridged network. These networks have a separate vLAN on the device underlying the other networks, which is created when the network is configured. VMs are then bridged into this vLAN.

With this client network type, PVC does no management of the network. This is left entirely to the administrator. It requires switch support and the configuration of the vLANs on the switchports of each node's physical interfaces before enabling the network.

VXLAN (managed) Client Networks

The second type of client network is the managed VXLAN network. These networks make use of BGP EVPN, managed by route reflection on the coordinators, to create virtual layer 2 Ethernet tunnels between all nodes in the cluster. VXLANs are then run on top of these virtual layer 2 tunnels, with the active primary PVC node providing routing, DHCP, and DNS functionality to the network via a single IP address.

With this client network type, PVC is in full control of the network. No vLAN configuration is required on the switchports of each node's physical interfaces, as the virtual layer 2 tunnel travels over the cluster layer 3 network. All client network traffic destined for outside the network will exit via the upstream network interface of the active primary coordinator node. NOTE: This may introduce a bottleneck and tromboning if there is a large amount of external and/or inter-network traffic on the cluster. The administrator should consider this carefully when sizing the cluster network.

Other Client Networks

Future PVC versions may support other client network types, such as direct-routing between VMs.

Node Layout: Considering how nodes are laid out

A production-grade PVC cluster requires 3 nodes running the PVC Daemon software. 1-node clusters are supported for very small clusters, home labs, and testing, but provide no redundancy; they should not be used in production situations.

Node Functions: Coordinators versus Hypervisors

Within PVC, a given node can have one of two main functions: it can be a "Coordinator" or a "Hypervisor".

Coordinators

Coordinators are a special set of 3 or 5 nodes with additional functionality. The coordinator nodes run, in addition to the PVC software itself, a number of databases and additional functions which are required by the whole cluster. An odd number of coordinators is always required to maintain quorum, though there are diminishing returns when creating more than 3. These additional functions are:

0. The Zookeeper database containing the cluster state and configuration
1. The DNS aggregation Patroni PostgreSQL database containing DNS records for all client networks
2. The FRR EBGP route reflectors and upstream BGP peers

In addition to these functions, coordinators can usually also run all other PVC node functions.

The set of coordinator nodes is generally configured at cluster bootstrap, initially with 3 nodes, which are then bootstrapped together to form a basic 3-node cluster. Additional nodes, either as coordinators or as hypervisors, can then be added to the running cluster to bring it up to its final size, either immediately or as the needs of the cluster change.

The Primary Coordinator

Within the set of coordinators, a single primary coordinator is elected at cluster startup and as nodes start and stop, or in response to administrative commands. Once a node becomes primary, it will remain so until it stops or is told not to be. This coordinator is responsible for some additional functionality in addition to the other coordinators. These additional functions are:

0. The floating IPs in the main networks
1. The default gateway IP for each managed client network
2. The DNSMasq instance handling DHCP and DNS for each managed client network
3. The API and provisioner clients and workers

PVC gracefully handles transitioning primary coordinator state, to minimize downtime. Workers will continue to operate on the old coordinator if available after a switchover and the administrator should be aware of any active tasks before switching the active primary coordinator.

Hypervisors

Hypervisors consist of all other PVC nodes in the cluster. For small clusters (3 nodes), there will generally not be any non-coordinator nodes, though adding a 4th would require it to be a hypervisor to preserve quorum between the coordinators. Larger clusters should generally add new nodes as Hypervisors rather than coordinators to preserve the small set of coordinator nodes previously mentioned.

Geographic redundancy

PVC supports geographic redundancy of nodes in order to facilitate disaster recovery scenarios when uptime is critical. Functionally, PVC behaves the same regardless of whether the 3 or more coordinators are in the same physical location, or remote physical locations.

When using geographic redundancy, there are several caveats to keep in mind:

• The Ceph storage subsystem is latency-sensitive. With the default replication configuration, at least 2 writes must succeed for the write to return a success, so the total write latency of a write on any system may be equal to the maximum latency between any two nodes. It is recommended to keep all PVC nodes as "close" latency-wise as possible or storage performance may suffer.

• The inter-node PVC networks must be layer-2 networks (broadcast domains). These networks must be spanned to all nodes in all locations.

• The number of sites and positioning of coordinators at those sites is important. A majority (at least 2 in a 3-coordinator cluster, or 3 in a 5-coordinator) of coordinators must be able to reach each other in a failure scenario for the cluster as a whole to remain functional. Thus, configurations such as 2 + 1 or 3 + 2 split across 2 sites do not provide full redundancy, and the whole cluster will be down if the majority site is down. It is thus recommended to always have an odd number of sites to match the odd number of coordinators, for instance a 1 + 1 + 1 or 2 + 2 + 1 configuration. Also note that all hypervisors much be able to reach the majority coordinator group or their storage will be impacted as well.

If these requirements cannot be fulfilled, it may be best to have separate PVC clusters at each site and handle service redundancy at a higher layer to avoid a major disruption.

Example Configurations

This section provides diagrams of 3 possible node configurations. These diagrams can be extrapolated out to almost any possible configuration and number of nodes.

Basic 3-node cluster

Above: A diagram of a simple 3-node cluster; all nodes are coordinators, single 1Gbps network interface per node, collapsed cluster and storage networks

Mid-sized 8-node cluster with 3 coordinators

Above: A diagram of a mid-sized 8-node cluster with 3 coordinators, dual bonded 10Gbps network interfaces per node

Large 17-node cluster with 5 coordinators

Above: A diagram of a large 17-node cluster with 5 coordinators, dual bonded 10Gbps network interfaces per node for both cluster/upstream and storage networks