What is Cluster?

A cluster is a logical group of hosts (compute nodes). In a real data center, a cluster usually maps to a rack.

When you plan a cluster, note that:
  1. All hosts in the same cluster must be installed with the same operating system.
  2. All hosts in the same cluster must have the same network configuration.
  3. All hosts in the same cluster must be able to access the same primary storage.
  4. Before a cluster can provide VM services, the cluster must have a primary storage and an L2 network attached.
  5. The scale of a cluster, which is the maximum number of hosts that the cluster can contain, is not enforced.

The relationship between a typical cluster and its associated resources is as follows.

Cluster | Zone

You can create more than one cluster in a zone, and allocate newly created hosts to different clusters as needed.

Cluster | Primary Storage and L2 Network

You can attach primary storages and L2 networks to or detach them from a cluster. The following diagram shows the relationship between cluster and primary storage, L2 network.
Figure 1. Relationship Between Cluster and Primary Storage, L2 Network


Note:
When you attach a primary storage and an L2 network to a cluster, note that:
  1. Cluster | Primary Storage
    • A primary storage can be attached to one or more clusters.
    • A cluster can have one or more primary storages attached.
      The following are primary storages of the same type that a cluster can have:
      • A cluster can have one or more LocalStorage primary storages attached.
      • A cluster can have one or more NFS primary storages attached.
      • A cluster can have one or more SharedBlock primary storages attached.
      • A cluster can have one SharedMountPoint (SMP) primary storages attached.
      • A cluster can have only one Ceph primary storage attached.
      • A cluster can have only one AliyunNAS primary storage attached.
      • A cluster can have only one AliyunEBS primary storage attached.
      The following are combinations of primary storages that a cluster can have:
      • A cluster can have both a LocalStorage and an NFS primary storage attached.
      • A cluster can have both a LocalStorage and an SMP primary storage attached.
      • A cluster can have both a LocalStorage and a SharedBlock primary storage attached.
      • A cluster can have both a Ceph and a SharedBlock primary storage attached.
      • A cluster can have both a Ceph and more than one SharedBlock primary storage attached.
      The following table lists the relationship between primary storages and a cluster.
      Table 1. Relationship Between Primary Storage and Cluster
      Primary Storage Cluster
      LocalStorage A cluster can have one or more LocalStorage primary storages attached.
      NFS A cluster can have one or more NFS primary storages attached.
      SharedBlock A cluster can have one or more SharedBlock primary storages attached.
      SMP A cluster can have one SMP primary storage attached.
      Ceph A cluster can have only one Ceph primary storage attached.
      AliyunNAS A cluster can have only one AliyunNAS primary storage attached.
      AliyunEBS A cluster can have only one AliyunEBS primary storage attached.
      LocalStorage + NFS A cluster can have one LocalStorage and one NFS primary storage attached.
      LocalStorage + SMP A cluster can have one LocalStorage and one SMP primary storage attached.
      LocalStorage + SharedBlock A cluster can have one LocalStorage and one SharedBlock primary storage attached.
      Ceph + SharedBlock
      • A cluster can have one Ceph and one SharedBlock primary storage attached.
      • A cluster can have one Ceph and multiple SharedBlock primary storages attached.
    • When you attach multiple LocalStorage primary storages to a cluster, partition the corresponding URLs on the hosts before you add hosts and primary storages, and make sure that each LocalStorage is deployed on an exclusive logical volume or physical disk.
    • A primary storage can be accessed by all hosts in the cluster to which the primary storage belongs.
    • If a primary storage cannot be accessed by hosts in the cluster due to network typology changes in the data center, you can detach the primary storage from the cluster.
  2. Cluster | L2 Network
    • A cluster can have one or more L2 networks attached. Also, an L2 network can be attached to one or more clusters.
    • A cluster can have a VXLAN pool attached. The VNIs in the VXLAN pool can be used to create different VXLAN networks.
    • One NIC can be used to create only one NoVlan network.
    • For VLAN networks, different VLAN IDs represent different L2 networks.
    • If hosts in a cluster no longer exist in the layer 2 broadcast domain of an L2 network due to network typology changes in the data center, you can detach the L2 network from the cluster.

Cluster | Backup Storage

No direct dependency exists between a cluster and a backup storage. A backup storage can provide services for multiple clusters.
Note:
  • The primary storages and backup storages attached to the same cluster are associated with each other.
  • A Ceph primary storage can work with ImageStore backup storages.
  • The following table lists the relationship between primary storages (PS) and backup storages (BS).
    Table 2. Relationship Between PS and BS
    PS/BS ImageStore SFTP Ceph
    LocalStorage ○ ○ ×
    NFS ○ ○ ×
    SMP ○ ○ ×
    Ceph ○ × ○
    SharedBlock ○ × ×