The SAN Volume Controller solution is based on a group of virtualization concepts. Before setting up your SAN Volume Controller environment, you should understand the concepts and the objects in the environment.
Each SAN Volume Controller is a single processing unit called a node. Nodes are deployed in pairs to make up a cluster. A cluster can consist of one to four pairs of nodes. Each pair of nodes is known as an I/O group and each node can be in only one I/O group.
Virtual disks (VDisks) are logical disks that are presented by the clusters. Each VDisk is associated with a particular I/O group. The nodes in the I/O group provide access to the VDisks in the I/O group. When an application server performs I/O to a VDisk, it can access the VDisk with either of the nodes in the I/O group. Because each I/O group has only two nodes, the distributed cache is only two-way.
Each node does not contain any internal battery backup units and therefore must be connected to an uninterruptible power supply, which provides data integrity in the event of a cluster wide power failure. In such situations, the uninterruptible power supply maintains power to the nodes while the contents of the distributed cache are dumped to an internal drive.
The nodes in a cluster see the storage that is presented by back-end disk controllers as a number of disks, known as managed disks (MDisks).
Each MDisk is divided into a number of extents which are numbered, from 0, sequentially from the start to the end of the MDisk. MDisks are collected into groups, known as MDisk groups.
Each VDisk is made up of one or two VDisk copies. Each VDisk copy is an independent physical copy of the data that is stored on the VDisk. A VDisk with two copies is known as a mirrored VDisk. VDisk copies are made out of MDisk extents. All the MDisks that contribute to a particular VDisk copy must belong to the same MDisk group.
A VDisk can be space-efficient. This means that the capacity of the VDisk as seen by host systems, called the virtual capacity, can be different from the amount of storage that is allocated to the VDisk from MDisks, called the real capacity. Space-efficient VDisks can be configured to automatically expand their real capacity by allocating new extents.
At any one time, a single node in the cluster can manage configuration activity. This node is known as the configuration node and manages a cache of the information that describes the cluster configuration and provides a focal point for configuration.
For a SCSI over fibre-channel connection, the nodes detect the fibre-channel ports that are connected to the SAN. These correspond to the worldwide port names (WWPNs) of the fibre-channel host bus adapters (HBAs) that are present in the application servers. You can create logical host objects that group WWPNs that belong to a single application server or to a set of them.
For an iSCSI over Ethernet connection, the iSCSI qualified name (IQN) identifies the iSCSI target (destination) adapter. Host objects can have both IQNs and WWPNs.
SAN Volume Controller hosts are virtual representations of the physical host systems and application servers that are authorized to access the cluster VDisks. Each SAN Volume Controller host definition specifies the connection method (SCSI over fibre-channel or iSCSI over Ethernet), the fibre-channel port or Ethernet IP address, and the VDisks that the host applications can access.
The cluster provides block-level aggregation and volume management for disk storage within the SAN. The cluster manages a number of back-end storage controllers and maps the physical storage within those controllers into logical disk images that can be seen by application servers and workstations in the SAN. The SAN is configured in such a way that the application servers cannot see the back-end physical storage. This prevents any possible conflict between the cluster and the application servers both trying to manage the back-end storage.